Unfortunately, it is not possible to add the controller to a smartthings network as they are both primary coordinators and you can only have one of those.
You can probably add the individual thermostats and use smartthings instead of the maestro smart controller. But you can’t connect one thermostat to both.
See the following discussion:
[RELEASE] Stelpro releases its Maestro Line-Voltage Thermostat: The SMT402