Mopeds do not work that way!
Well.. they do, or would.. but um..
I think it would be a bad idea to do it on the drive chain, there's definatly a reason motorcycles don't use derailers..
As far as the pedal side, you'd have to fab up a new swing arm, but it should be doable.
And using a bike rear wheel (especially axle), isn't a safe idea...
I'll admit the thought of adding gears so that I could assist my moped at higher speeds has crossed my mind, but.. I dono, just not something I thing I'd want to do to my bike.