CustomImporter.cs

using System.Reflection;
using UnityEditor;
using UnityEditor.Experimental.AssetImporters;
using UnityEngine;

    [CustomEditor(typeof(TextureImporter))]
    [CanEditMultipleObjects]
    //https://github.com/Unity-Technologies/UnityCsReference/blob/9034442437e6b5efe28c51d02e978a96a3ce5439/Editor/Mono/ImportSettings/TextureImporterInspector.cs
    internal class TextureImporterEditor : AssetImporterEditor
    {
        #region Original texture importer editor support
        private readonly Assembly editorAssembly = Assembly.GetAssembly(typeof(AssetImporterEditor));

        private AssetImporterEditor originalEditor;

        public override void OnEnable()
        {
            Editor origAsEditor = originalEditor as Editor;
            CreateCachedEditor(targets, editorAssembly.GetType("UnityEditor.TextureImporterInspector"), ref origAsEditor);
            originalEditor = origAsEditor as AssetImporterEditor;
            originalEditor.OnEnable();
        }
        public override void OnDisable()
        {
            originalEditor.OnDisable();
        }
        public override bool showImportedObject => false;

        public override bool HasModified()
        {
            return originalEditor.HasModified();
        }
        protected override void ResetValues()
        {
            callProtectedMethod(originalEditor, nameof(ResetValues));
        }
        protected override void Apply()
        {            
            callProtectedMethod(originalEditor, nameof(Apply));
        }
        private void callProtectedMethod(object target, string methodName)
        {            
            //target.GetType().GetMethod(methodName, BindingFlags.NonPublic | BindingFlags.Instance).Invoke(target, null);
        }
        #endregion

        bool showAdvancedImporter;

        public override void OnInspectorGUI()
        {
            originalEditor.OnInspectorGUI();
            //do your custom UI
        }
    }

Hi Julien, i never tried. But perhaps if you make the CustomEditor attribute use the ModelImporter type (https://docs.unity3d.com/ScriptReference/ModelImporter.html) it would work!