Using Channel 6 to switch relay pin - CHDK

I want to use my quadrocopter with a Canon in CHDK-Mode, trigger cam using USB-Cable.

My only switch left ist Channel 6. How can i use this channel to switch Relay Pin 9? Iam on APM 2.6 , SpektrumDX4e in PPM-Mode.

One idea would be to “simulate” CH6 to be CH8, for 8 you can set relay on off

The Ardupilot pages have plenty of information here

Thank you for your reply.
I know all these pages, but i cant find the answer to my problem there. Would you be so kind and point me to the right direction (by the way: i searched 3 hours for a solution, on the whole net) ,