Is it possible to run XWA with a gamepad? (X-box controller)

If you properly configure the gamepad through the "Game Controllers" dialog in Control Panel, it should. You may also need to specify the gamepad by clicking the "Advanced" button in that dialog and selecting the gamepad from the drop-down list. You really need a joystick, though, to get the true "feel" of a space sim. ;)

Why you would want to use a gamepad for a flightsim is beyond me. This isn't some console game with your veiw fixed to a chase camera veiw