I found out more about how people are using the DSC alarm panel with SmartThings.
You fit a supported DSC panel, you fit inside the panel a third-party module which adds a network interface and a web interface for the entire DSC alarm system. You can in theory stop there and just use that web interface to monitor and set the alarm. The next step is to run a python app on a suitable computer Mac, Linux even a RasberryPi, this acts as a server for SmartThings to talk to. Finally you use a SmartThings app to talk to the server.
With this it is still a traditional alarm system but has a link via the module to the LAN.
There are several threads in these forums discussing this, see DSC + Envisalink 4 + smartthings-dsc-alarm-server as a starting point.