Additional information is needed to respond to your request. What kind of devices do you want to add to your fire alarm system - smoke detectors, heat detectors, pull stations, audio/visual annunciation?
Basic fire alarm systems typically use detectors and pull stations with contact outputs. These contacts are monitored to determine when they change status from opened-to-closed or closed-to-open condition. Also, a good fire alarm system will also monitor the status of the system wiring to verify the integrity of the conductors linking the fire alarm system devices.
I realize this is not especially helpfull, but there is a final issue. The life/safety building and fire codes of virtually every state in the USA requires fire alarm systems to be listed systems. This means that they have been tested by the Underwriters Laboratories or a similar testing organization. this is intended to assure the public that we are protected by fire alarm sysems that are trust worthy. Therefore, the final question is: What do you intend to do with this system once it is complete?