Extend ParticleSystem editor with custom inspector GUI

CustomParticleSystemEditor.cs

using System.Reflection;
using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(ParticleSystem))]
public class CustomParticleSystemEditor : Editor
{
    private Editor _builtInEditor;

    private void OnEnable()
    {
        var builtInEditorType = typeof(UnityEditor.Editor).Assembly.GetType("UnityEditor.ParticleSystemInspector");
        _builtInEditor = Editor.CreateEditor(targets, builtInEditorType);
    }

    private void OnDisable()
    {
        var disableMethod = _builtInEditor.GetType().GetMethod("OnDisable", BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
        disableMethod!.Invoke(_builtInEditor, null);
        DestroyImmediate(_builtInEditor);
    }

    public override void OnInspectorGUI()
    {
        // Custom GUI
        GUILayout.Button("my button");

        // Built-in GUI
        _builtInEditor.OnInspectorGUI();
    }
}