I was doing some research a while,back about having any old alarm and using the fibaro relays inline to pickup the PIR data and connect to smartthings. Fibaro - Universal Binary Sensor - Does it work with Smart Things
I also looked at a Honeywell Galaxy Flex 20 which supports a programmable relay board which could feed the fibaro relays or the door entry device that has wires you could use tag in to on the post hard wired alarm logic. If you give it a wired internet connection too it has its own app, so there might be an api to,hook in to.