ModelImporter

ModelImporterEditor.cs

using System.Reflection;

using UnityEditor;
using UnityEditor.AssetImporters;

using UnityEngine;

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

    private AssetImporterEditor originalEditor;

    public override void OnEnable()
    {
        originalEditor = CreateEditor(targets, editorAssembly.GetType("UnityEditor.ModelImporterEditor")) as AssetImporterEditor;
        originalEditor.OnEnable();

        base.OnEnable();
    }
    public override void OnDisable()
    {
        originalEditor.OnDisable();        
        base.OnDisable();
    }
    public override bool showImportedObject => false;



    public override bool HasModified()
    {
        return originalEditor != null && 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;
    
    protected override void OnHeaderGUI()
    {
        GUILayout.Label("HEADER");
    }
    public override void OnInspectorGUI()
    {
        GUILayout.Label("TEST");
        originalEditor.OnInspectorGUI();
        //do your custom UI

        ApplyRevertGUI();
    }
}