Why can't you get access to the FA panel?
Adding another component into the signal circuit that is not part of the Fire Alarm system is WRONG. The supervisory current will be affected. Life Safety Systems should not be modified or added to without the manufacturer's approval. If something was to happen to that signal circuit or the whole system and somebody died in a fire because they had no warning, I wouldn't want to be in your shoes.
Also, what happens when they test the Fire Alarm during maintenance? The security system will go off. If you don't have access to the FA Panel, do the people testing the FA system have access and the codes to shut off the security system?
