Hi Dorian,
What you want to do isn’t supported. You can always modify the firmware to do it as it is open source, but if you are connecting an Arduino I would suggest you to use MAVLink and send commands through it. If you search Google you’ll find lots of info, including a topic here in the forum:
http://discuss.ardupilot.org/t/establishing-mavlink-connection-between-arduino-and-pixhawk/13237