I have never herd of anyone trying to make more than a 3 speed tranny. Fist of all it would have to be huge and second of all it would weigh a ton. It would be pretty cool though, going from first to second sounds awesome but going from first to second to third to fourth to fifth would be sweet!

The only way you could really make one would to make an aluminum case. I don't know what gears you would use though. Just my thoughts.