I added expansion cabinets to two different 25.40 systems a couple years back and there was no keycode setting I was aware of. You just need to make sure you have the right Fiber Daughter and Reciever cards.
One word or caution. Make sure all your solder connections on the power supply and Receiver card are not too long. When I did one expansion addition in 2002, the solder make contact between the power supply and new Fiber card. Never seen a PBX start on fire before, and hope never to see it again. It was rather small and quick, and it went out after I turned the Power supply off.
The best part of this story was I powered it back on, after checking all the solder connections and the Fiber card, and outside of being a little black, both cards looked OK. The units are still running just fine today.
Scott M.