hey ben this can be done with stock rims and thats what i highly recommend,cause they are stronger then just about any aftermarket rim.
but you will have to pull out the air stem and put it in backwards and then drill a hole on the otherside of the rim and intsall another valve the correct way.
thats what i did on mine and just about every quad i've had and alot of peoples quads i know get the same mod. not very hard to do at all.