Unfortunately, we don't have an example of a HID joystick for the FX. We do have a HID example for FX that implements a composite device. This composite device functions as a limited mouse and keyboard. You can use the development board's push buttons to actuate mouse and keyboard events. You could potentially use this as a starting point.
The appnote entitled: Creating a Sample EZ-USB Composite HID Device and example is available on our website.