I drilled my rims' shrader holes out to 3/8" so I could use truck valve extenders with my Super Tubes. It is about 1/16" difference from the original hole. I have done it before, but it is sort of risky. It is best to use a rat tail file if you have one, instead. If the bit doesn't cut well (too slow/too much pressure/too dull), it can flake a big chunk out of your rim or crack it. This is not hard to do on fat cruiser rims, but might not be possible on road rims.
Hopefully this and a fatter tire will end the plague of thorn flats.