As always this newsgroup comes through....Thanks for all the advice! I won't be attempting to interface the system now, and I'm going to try to convert the whole system over to 12v Napco FW-4W detectors, probably just capping off the 110 wiring outside the existing boxes, and diconnecting it at the main panel. Lot more work, but I really want the smokes to be monitored, so looks like that's my only safe option.
Thanks Again to All who provided advice!