I believe interconnected smoke alarms are not really "fire alarm systems" (hence they don't qualify for the AFCI exception). A regular EC can connect them but modifying or adding something that is not in the manufacturer's instructions is a 110.3(B). I can think of a couple ways you might be able to trick them into going off from a 120v signal (relay, 9v wall wart etc) but it is probably a listing violation if you don't have guidance from the manufacturer.
