Frederic,

I tried this once, just out of curiosity I put a 9 speed wheel onto an old bike with Suntour friction stem shifters with the normal 7 speed drivetrain (cranks, derailers, and chain) in place. I only pedaled it while on the stand, running through the rear gears from top to bottom (with a minor adjustment on the limits) and it covered the gear range, although the chain (7-8 speed) seemed to have a hard time settling in on the cogs and jumped back and forth a bit. I think with a 9 speed chain and the right derailer pulleys it would work fine with normal stem shifters (as long as your derailer can cover the additional dish in the wheel). It definitely wouldn't work with indexed shifters.

CMC