Maybe a bit late on this one, but I play Nexuiz using a Playstation DualShock.
Assuming that you've got the gamepad connected to and recognised by your PC, then I'd recommend downloading ControlMK, which allows you to map keyboard and mouse commands to the joypad buttons/sticks.
If it's of any use, I'd be happy to send you a copy of the configuration that I use (ControlMK can export profiles, failing that, I could write it up). Of course, if you're not running Windows (I note that it doesn't seem to be a popular OS, round here!), this is all about as much use as a chocolate teapot.
Quick word of caution: playing with a gamepad will never be as quick or accurate as mouselook, so experienced players will probably deal out massive ass kickings (or perhaps that's just me)!
