From what I understand, you want to make an external trim wheel. This is possible by using a simple pot meter. However, when you say wiring it to the original pot, do you mean that you want it to control that one? This is not possible. I would just get another pot for the trim, and connect it to the yoke. I am not sure how you should connect it to the yoke, since I don't have any USB game devices, however, USB does have enough axis to allow an etra one. A gameport has a limit of 4 axis, because there just arent enough pins left after 4.
Anyway, you can take the 5 volt from any other pot, by just adding another piece of wire going to the new pot. then the other wire coming back and going to the PC will have to be connected to the USB somehow. Fs has sepparate axis for trim so that is no problem.