you mentioned "alarm" or were you refering to "smoke detectors"??
First disregarding local regulations for feeding the smoke detectors.
If you are installing multi-station smoke detectors and they interconnect via 3 wires, (14/3NMB) you can disconnect the non AFCI feed and back feed from the new AFCI.
While that is the most obvious solution I was looking at other options.
When I use the term Smoke
Detector I think of the Sensor, attachment #2, used in a Central Fire Alarm System. A Smoke
Alarm is what I find in SFD's that interconnect, attachment #1
In my opening post I should have used the term remodel instead of addition. This is a basement that is being finished. There is a Smoke Alarm dead ended in the basement just a few feet away from what will be the bedroom. The panel is not marked well and I don't know which of the existing smokes is the feed.
I have been asked to bid this job so I'm trying to cover my bases to keep from getting burned, pardon the pun, on $$$.