The file is '.kde/share/config/kdesktop_custom_menu1' (or whatever number) and, based on its simplicity and resistance to any meaningful edit, along with the limitations of the GUI widgets, I'd say it isn't doable without genuine hacking. But I could be wrong, obviously. There may be a way to specify 'menu' or to 'include' another file but, like I say, it's a key=value format and none of the keys (if any) are documented.
It's this kind of hypercomplexity of overall structures - such as the main menu - coupled with astonishing lapses of power and configurability - such as the custom menu - that usually has me dumping Knome pretty quickly.
Not to mention the equivalent of thousand-page books to document the things that still don't really tell you anything.