Can I create custom EditorWindow that shows up in the Add Tab menu?

I am wondering how is it possible to list my own EditorWindows in the Add Tab menu. (in the hamburger-menu at top right of panels)

No you can’t. Well you might be able to add more elements to the Add Tab sub menu using the IHasCustomMenu interface in your editor window. However those would only be added to your own editor window but not others.

The issue is that the GenericMenu that is used to display the menu is hardcoded in the DockArea of the ContainerWindow. The Add Tab menu is populated here, The GetPaneTypes method that is used internally also returns a hardcoded list of editor window types. So even with reflection we have no way to add something to that list.

As i said it may be possible for your editorwindow to modify the menu for that window but not generally for all menus.

You can use this bit of code which should work :