It’s not possible using umenu, so you would have to construct your own menu. One method would be to use a series of textbuttons inside of a named bpatcher, and another button in the main patch. When you click the button in the main patch, send the message "script show [the bpatcher’s name]" to thispatcher, and when you click the button again, send the message "script hide [the bpatcher’s name]." Unfortunately, it might be complicated for a dynamic number of menu items.