Intentionally mis-use the USB port on an Arduino?
So the usb port is being as a general connector, not as a USB standard device? If so, just get a usb port or cable and wire it to the Arduino outputs.

If it's being used as an actual usb slave device, then that's alot more complicated.

Either way, if you want to use the existing usb port, you would need an arduino that uses a AVR ic as the usb port, and reprogram it so that it can do two functions (not simple). The older arduinos use a FTDI FT232R, a dedicated usb to serial IC that would not work well with what you want to do.