That's the kind of stuff one can find in google. I'll also send you some pictures of the inside of my foot controller, that might help.
As i said in another thread i recommend buying a relatively good gamepad, especially if one wants to use pots (as in an expression pedal for instance). What happens with cheap gamepads is that the data [hid] gets is not linear any more (it grows from 0 to say 90 as it should, then it gets stuck around 125 for a while, jumps to 150 and grows normally up to 255). This implies that your expression pedal will behave in a strange way. I thus strongly recommend testing the gamepad first with [hid] before hacking.