Modifying Menus

When you insert a new item in an existing menu, you probably want to put it in a particular place among the menu's existing items. If you use define-key to add the item, it normally goes at the front of the menu. To put it elsewhere in the menu, use define-key-after:

— Function: define-key-after map key binding &optional after

Define a binding in map for key, with value binding, just like define-key, but position the binding in map after the binding for the event after. The argument key should be of length one—a vector or string with just one element. But after should be a single event type—a symbol or a character, not a sequence. The new binding goes after the binding for after. If after is t or is omitted, then the new binding goes last, at the end of the keymap. However, new bindings are added before any inherited keymap.
Here is an example:
          (define-key-after my-menu [drink]
            '("Drink" . drink-command) 'eat)
makes a binding for the fake function key <DRINK> and puts it right after the binding for <EAT>.
Here is how to insert an item called ‘Work’ in the ‘Signals’ menu of Shell mode, after the item break:
          (define-key-after
            (lookup-key shell-mode-map [menu-bar signals])
            [work] '("Work" . work-command) 'break)

22.17.7 Modifying Menus