how to add to village farms in a mod/datapack compatible way that can stack without overwriting each other.

addModdedCropsToVillageFarms.java

    // Note: Lithostitch mod allows you to add to processor lists easily! No code needed. 
    // https://github.com/Apollounknowndev/lithostitched/wiki/Worldgen-Modifier-Types#add_processor_list_processors

    // This gist code is public domain. Take it! Steal it! AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!!!!!!!!!!!!
    
    // 1.18.2+
    public ExampleMod() {
            MinecraftForge.EVENT_BUS.addListener(this::addNewVillageCrop);
    }
    
    public void addNewVillageCrop(final ServerAboutToStartEvent event) {
        // Dynamic registry of processor lists. We will edit the processor lists from here as this is what the world will actually use.
        Registry<StructureProcessorList> processorListRegistry = event.getServer().registryAccess().registry(Registry.PROCESSOR_LIST_REGISTRY).orElseThrow();
       
        // Create a new processor to add to the processor lists
        StructureProcessor newCropProcessor = new RuleProcessor(List.of(new ProcessorRule(
                // We replace the vanilla Wheat block with Sweet Berry Bush 50% of the time.
                // Note, Potatoes and Beetroot will also be avaliable to be replaced too by our processor.
                new RandomBlockMatchTest(Blocks.WHEAT, 0.5F),
                // Location predicate. Keep this as always true for most use-cases.
                AlwaysTrueTest.INSTANCE,
                // The modded block to use.
                Blocks.SWEET_BERRY_BUSH.defaultBlockState()
        )));
        
        // Now add new rule processor to vanilla crop processors
        addNewRuleToProcessorList(new ResourceLocation("minecraft:farm_plains"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("minecraft:farm_savanna"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("minecraft:farm_snowy"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("minecraft:farm_taiga"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("minecraft:farm_desert"), newCropProcessor, processorListRegistry);
        
        // Can target other mod's processor lists
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/badlands/crop_replacement"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/birch/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/dark_forest/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/giant_taiga/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/jungle/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/mountains/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/oak/crop_randomizer"), newCropProcessor, processorListRegistry);
        addNewRuleToProcessorList(new ResourceLocation("repurposed_structures:villages/swamp/crop_randomizer"), newCropProcessor, processorListRegistry);
    }

    private static void addNewRuleToProcessorList(ResourceLocation targetProcessorList, StructureProcessor processorToAdd, Registry<StructureProcessorList> processorListRegistry) {
        processorListRegistry.getOptional(targetProcessorList)
                .ifPresent(processorList -> {
                    // The list of processor is an immutable list.
                    // So we will create a new list, add to it, and set the field to that new list.
                    List<StructureProcessor> newSafeList = new ArrayList<>(processorList.list());
                    newSafeList.add(processorToAdd);
                    processorList.list = newSafeList; // Use an Access Transformer or Accessor Mixin to set the private list field.
                });
    }
    
    
////////////////////////////////////////////////////////

    // 1.16.5 - (this old way is outdated and probably entirely unnecessary. Try the method done above instead and see if that works in 1.16.5)
    public ExampleMod() {
            MinecraftForge.EVENT_BUS.addListener(this::addNewVillageCrop);
            FMLJavaModLoadingContext.get().getModEventBus().addListener(this::createProcessors);
    }
      
      
    // We will create the processor in FMLCommonSetupEvent as we know our 
    // block will be created and registered by the time this event fires.
    public static RuleStructureProcessor MODDED_CROP_PROCESSOR;

    public void createProcessors(final FMLCommonSetupEvent event) {
        MODDED_CROP_PROCESSOR = new RuleStructureProcessor(ImmutableList.of(
            new RuleEntry(
                    // We replace the vanilla Wheat block with Sweet Berry Bush 50% of the time.
                    // Note, Potatoes and Beetroot will also be avaliable to be replaced too by our processor.
                    new RandomBlockMatchRuleTest(Blocks.WHEAT, 0.5F),
                    // Location predicate. Keep this as always true for most use-cases.
                    AlwaysTrueRuleTest.INSTANCE,
                    // The modded block to use.
                    Blocks.SWEET_BERRY_BUSH.defaultBlockState()
            )
        ));
    }


    /**
     * Appends the given processor to the end of the targeted pieces in the targeted pools.
     * We will call this in addNewVillageCrop method further down to add to every village farm pool entry.
     * 
     * Note: This is an additive operation which means multiple mods can do this and they stack with each other safely.
     */
    private static void addCropToPoolElement(MutableRegistry<JigsawPattern> templatePoolRegistry, ResourceLocation poolRL, List<ResourceLocation> nbtPieceRLList) {
        JigsawPattern pool = templatePoolRegistry.get(poolRL);
        if (pool == null) return;

        // AccessTransformer to make JigsawPattern's templates field public for us to see.
        // public net.minecraft.world.gen.feature.jigsaw.JigsawPattern field_214953_e #templates
        pool.templates.forEach(entry -> {
            if (entry instanceof SingleJigsawPiece) {
                SingleJigsawPiece jigsawPiece = (SingleJigsawPiece) entry;

                // AccessTransformer to make SingleJigsawPiece's template field public for us to see.
                // public net.minecraft.world.gen.feature.jigsaw.SingleJigsawPiece field_236839_c_ #template
                if (jigsawPiece.template.left().isPresent()) {

                    // Found the piece entry we want to modify.
                    ResourceLocation currentNBTPiece = jigsawPiece.template.left().get();
                    if (nbtPieceRLList.stream().anyMatch(currentNBTPiece::equals)) {

                        // AccessTransformer to make SingleJigsawPiece's processors field public and non-final for us to see and swap out.
                        // public-f net.minecraft.world.gen.feature.jigsaw.SingleJigsawPiece field_214862_b #processors
                        StructureProcessorList originalStructureProcessorList = jigsawPiece.processors.get();

                        // Make a mutable list as this is an immutable list originally
                        List<StructureProcessor> mutableProcessorList = new ArrayList<>(originalStructureProcessorList.list());

                        // Add our processor to the end so it runs after everything else
                        mutableProcessorList.add(MODDED_CROP_PROCESSOR);
                        StructureProcessorList newStructureProcessorList = new StructureProcessorList(mutableProcessorList);

                        // Override the original field with our new instance. This is an additive operation and safe to stack with other mods
                        jigsawPiece.processors = () -> newStructureProcessorList;
                    }
                }
            }
        });
    }


    /**
     * We use FMLServerAboutToStartEvent as the dynamic registry exists now and all JSON worldgen files were parsed.
     * Mod compat is best done here.
     */
    public void addNewVillageCrop(final FMLServerAboutToStartEvent event) {
        MutableRegistry<JigsawPattern> templatePoolRegistry = event.getServer().registryAccess().registry(Registry.TEMPLATE_POOL_REGISTRY).get();

        // Adds the crop processor to all vanilla farm village pieces
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/plains/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/plains/houses/plains_large_farm_1"),
                new ResourceLocation("minecraft:village/plains/houses/plains_small_farm_1")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/plains/zombie/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/plains/houses/plains_large_farm_1"),
                new ResourceLocation("minecraft:village/plains/houses/plains_small_farm_1")
        ));

        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/snowy/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/snowy/houses/snowy_farm_1"),
                new ResourceLocation("minecraft:village/snowy/houses/snowy_farm_2")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/snowy/zombie/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/snowy/houses/snowy_farm_1"),
                new ResourceLocation("minecraft:village/snowy/houses/snowy_farm_2")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/savanna/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/savanna/houses/savanna_large_farm_1"),
                new ResourceLocation("minecraft:village/savanna/houses/savanna_large_farm_2"),
                new ResourceLocation("minecraft:village/savanna/houses/savanna_small_farm")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/savanna/zombie/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/savanna/houses/savanna_large_farm_1"),
                new ResourceLocation("minecraft:village/savanna/zombie/houses/savanna_large_farm_2"),
                new ResourceLocation("minecraft:village/savanna/houses/savanna_small_farm")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/taiga/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/taiga/houses/taiga_large_farm_1"),
                new ResourceLocation("minecraft:village/taiga/houses/taiga_large_farm_2"),
                new ResourceLocation("minecraft:village/taiga/houses/taiga_small_farm_1")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/taiga/zombie/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/taiga/houses/taiga_large_farm_1"),
                new ResourceLocation("minecraft:village/taiga/zombie/houses/taiga_large_farm_2"),
                new ResourceLocation("minecraft:village/taiga/houses/taiga_small_farm_1")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/desert/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/desert/houses/desert_large_farm_1"),
                new ResourceLocation("minecraft:village/desert/houses/desert_farm_1"),
                new ResourceLocation("minecraft:village/desert/houses/desert_farm_2")
        ));
        addCropToPoolElement(templatePoolRegistry, new ResourceLocation("minecraft:village/desert/zombie/houses"), ImmutableList.of(
                new ResourceLocation("minecraft:village/desert/houses/desert_large_farm_1"),
                new ResourceLocation("minecraft:village/desert/houses/desert_farm_1"),
                new ResourceLocation("minecraft:village/desert/houses/desert_farm_2")
        ));

        // Since we only use Resourcelocations to target where to add our processor, there's no hard dependency with other mod's code.
        // A simple check to see if the targeted mod exists is sufficient before actaully attempting to add.
        if (ModList.get().isLoaded("repurposed_structures")) {
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/badlands/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/badlands/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/badlands/houses/farm_2")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/badlands/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/badlands/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/badlands/houses/farm_2")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/birch/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/birch/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/birch/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/birch/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/birch/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/birch/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/dark_forest/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/dark_forest/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/dark_forest/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/dark_forest/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/dark_forest/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/dark_forest/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/giant_tree_taiga/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/giant_taiga/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/giant_taiga/houses/farm_2")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/giant_tree_taiga/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/giant_taiga/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/giant_taiga/houses/farm_2")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/jungle/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/jungle/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/jungle/houses/large_farm_2"),
                    new ResourceLocation("repurposed_structures:villages/jungle/houses/small_farm")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/jungle/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/jungle/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/jungle/zombie/houses/large_farm_2"),
                    new ResourceLocation("repurposed_structures:villages/jungle/houses/small_farm")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/mountains/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/mountains/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/mountains/houses/large_farm_2"),
                    new ResourceLocation("repurposed_structures:villages/mountains/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/mountains/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/mountains/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/mountains/zombie/houses/large_farm_2"),
                    new ResourceLocation("repurposed_structures:villages/mountains/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/mushroom/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/mushroom/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/mushroom/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/oak/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/oak/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/oak/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/oak/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/oak/houses/large_farm_1"),
                    new ResourceLocation("repurposed_structures:villages/oak/houses/small_farm_1")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:village/swamp/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/swamp/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/swamp/houses/farm_2")
            ));
            addCropToPoolElement(templatePoolRegistry, new ResourceLocation("repurposed_structures:villages/swamp/zombie/houses"), ImmutableList.of(
                    new ResourceLocation("repurposed_structures:villages/swamp/houses/farm_1"),
                    new ResourceLocation("repurposed_structures:villages/swamp/houses/farm_2")
            ));
        }
    }

Fixed again and thanks! I’m guessing you looked at my StructureTutorialMod for creating a new structure. But yeah, doing a reverse pillager outpost placing is smart for having a nearby building to villages without actually being connected. I’m imagining a village barn and stuff lol. Good luck with your mod!

I just checked, and yep I got inspiration from the StructureTutorialMod, I did not even realize that was also yours.