Errrm.. You cant add one, because you need the right stuff.
Ive seen adverts on ebay claiming to have a touch sensitive wheel.
you could try buying it, if it hasnt got a click wheel like the description, get a refund or somthing?
I dont think apple have copyright over a wheel that slides round with your finger.. lol
All I say is, you could try it, but im not sure.
If it works? Cant you take that wheel off, and the stuff and put it on your player now?
not sure.. you could try it?