Add explicit categories to HTML Imported posts

html-importer.php

<?php
// [file fragment]
// find this, around line 671
// ... and all the taxonomies...
		$taxonomies = get_taxonomies( array( 'public' => true ), 'objects', 'and' );
		foreach ( $taxonomies as $tax ) {
			if ( isset( $options[$tax->name] ) )
				wp_set_post_terms( $post_id, $options[$tax->name], $tax->name, false );
		}
		if ( isset( $customfieldtags ) )
			wp_set_post_terms( $post_id, $customfieldtags, 'post_tag', false );
		
		
		
// add this on a new line:
wp_set_post_terms( $post_id, 'category-ID', 'category', false );

// note that false replaces existing categories. Change to true to append instead. 
// Category IDs can be single, or an array or comma-separated string
// see https://codex.wordpress.org/Function_Reference/wp_set_post_terms