Rampy,
The basic idea is on the back of the wheel, on the axle, instead of having an encoder wheel, you'd just have a rubber wheel that rests on the top of the trackball. You spin the wheel and the trackball spins with it (but in the opposite direction).
You're probably right though. It wouldn't be that hard to make a hidden connector and switch to share the electronics for the spinner with a steering wheel. Since the optics are so inexpensive, it probably would make more sense to just wire it up instead of making a mechanical solution.
AX,
Like rampy suggested, obtaining the wheel and making a way to mount it to the CP are probably the bigger issues.
On the other hand, if I'm going to go through the trouble of making a steering wheel assembly, I might as well make a cockpit style driving cabinet, right?

Wade