I have integrated wired door sensors into Smart Things using an Arduino UNO and a Thing Shield. This required programming on the Arduino and some groovy code to create a custom DeviceType for the Arduino, as well as a SmartApp to multiplexe/demultiplex some virtual “contact sensor” devices. My house did not have an existing alarm system, but was pre-wired. The Arduino and Thing Shield cost about $60 total, and I have 6 doors integrated, and no batteries to ever change. I also monitor the temperature in my garage using the same Arduino. If I had bought all of the Zigbee/Zwave sensors to implement this solution, I would have spent $300 to $400 in parts (however the installation would have been simpler.) I spent under $100 and enjoyed the project immensely.
It has worked very well and has been very reliable thus far. Check out the forum discussion at Arduino SmartShield Garage Controller