I asked a similar question a couple months ago - check out the thread below:

http://www.poolforum.com/pf2/showthread.php?t=6522

As mentioned in the other thread, you might want to check the impeller for the 1-speed version of the pump to see if it's the same as the impeller for the 2-speed version of the pump. The operating manual might have an exploded view of the pump showing all its parts. Someone mentioned that the impellers are usually matched to the power of the motor.

If you DO do it, report back as I'd be curious how it works out for you.

Good luck!