French QWERTY keyboard for Nexus S
If your phone is rooted, it's possible to do this by modifying system files that govern the keyboard.  A much easier (and safer) way would be installing a 3rd-party keyboard replacement app that has a French QWERTY layout.

Out of keyboards I've personally used, I can recommend AnySoftKeyboard with French Language Pack. It includes both AZERTY and QWERTY (French-Canadian) layouts.