The easist way to do that would be to re-program the key mappings in the MAME tab menu. (At least that's how I would do it.)
I had a KE24 (before I fried it about 2 weeks ago.) You can program one key to send multiple keystrokes but not 2 keys to send one stroke.
(ie: start1 = "1" + "esc" + etc, not start1 + start2 = "esc")
You can do this easily through mame though. Just go to the main settings and do it there.
Smack