Several contact sensor reveal extra dry contacts when the case is open. People use them to add a zwave or zigbee antenna to all kinds of projects, including pressure mats, outdoor gate sensors, etc.
Here’s one project example using the Ecolink.
The Schlage sensor is another. @trotsky40 has used one to connect to his hardwired nonnetworked smoke alarms, for example.
Search the forums for “dry contact” and you’ll find other examples.
If you’re connecting directly to the panel, though, rather than one of the sensors, you may have to check the voltage compatibility. See this discussion:
Also if you search the forums for DSC, you’ll find descriptions of projects where people have connected to existing security systems in various ways.