you could do this:
1) Mount a BMX stem to the frame by using the clamp part, this will leave you with a bar sticking out the side.
2) cut the stem down so it is just long enough to fit the grip shifter
3) Clamp another stem over the grip shifter (you may need to shave some rubber off)
Wala! A stick shifter or you could get a lever arm shifter and just add an extension.
-Moto