Wired system with smart Things? (Existing wires, no devices--what now?)

Take a look at my ST_Anything project. I did the exact same thing as what you are proposing, and even automated my garage doors as well. I bought very inexpensive magnetic reed switches on Amazon to connect at each door using the alarm pre-wiring in my house. I bought them on Amazon. In the case of my house, the door opening were pre-drilled for the sensors with the wires just tucked inside each hole. Super simple to simply pull the wire out of the hole, attach the sensor (pay attention to proper diameter), and then push it into the hole. The magnet portion is simply installed in the edge of the door after drilling an appropriate sized hole, and pushing it in. It takes less than 15 minutes per door once you get the hang of it.

So, i was able to connect 4 house doors and 2 garage doors to ST for the cost of an Arduino UNO (or MEGA 2560 preferably), the ST ThingShield, a power supply, magnetic sensors, and relays (for the garage doors.) No batteries to change, ever!

In my github repository (and discussed on the following thread), you will find numerous examples of how my library has been used to integrate many different things with ST using an Arduino. Home alarm projects are the vast majority, so you should be able to get things going quickly if you choose this path.