It can be done, but you have to remove the detent balls and springs, use leverage to your advantage (which would mean a cable and shifter with ALOT of throw), and somehow detent the shifter to keep it from moving once a gear position has been reached.
Is it doable...Yes. Is it feasible or prudent with space limitations and cost/headaches associated........?
Is there a particular reason for wanting this (doubler, custom setup, etc.)?