It’s not officially supported by SmartThings. In fact adding SmartThings as a secondary to any other Z wave system is officially discouraged.
SmartThings strongly discourages adding the Hub to another Z-Wave network. We cannot offer support for disconnected Z-Wave devices or the inability to add devices through the Hub as a result of including the Hub into another Z-Wave network.
As far as why security companies treat it differently, that one is simple. Up until the spring of 2017, Z wave was not UL – approved for security transmissions. And now it is only approved for the very newest Devices which are using the S2 protocol, which does not include SmartThings. So it’s not that they see a big physical difference between the smart lock keypad and the wall panel inside the house, it’s that the message transmission from the lock didn’t meet security requirements.
All of that said, there are some community members who have been successful to some degree with adding SmartThings as a secondary controller to an ADT pulse panel, but it only works with the very newest models of the ADT pulse controller.
See the following thread: