It would not be too hard if you are good at electronics. I would tear apart a cheap joystick and replace the electronics with a PIC or AVR microcontroller. The HobbyCNC board needs 2 signals (direction and step) for each stepper motor. The step signal needs to pulse at the rate you want to turn the motor, and can be proportional to the joystick position. The output signals are just TTL level signals sent to a parallel port connector.
I plan on building one in the near future.