It’s technically possible to add a secondary controller like the Z stick to smartthings, but it doesn’t always work very well.
SmartThings is intended as a multi protocol platform, and for that reason it doesn’t use controller replication and a few of the other advanced Z wave features.
Fortunately, the zstick does have the option to request network updates, which does allow you to do a little more. It still gets pretty clunky.
(Note that the previous poster’s comment about virtual devices does not apply to Z wave devices. That method only works with zigbee devices like the lightbulb described because a zigbee device has its own unique ID that it tells the hub at the time of pairing. Z wave devices are given an ID by the hub. Also, the method the previous poster described doesn’t have to do with bringing in a device from a different network, as you are considering.)
@erocm1231 and a few other members have been using zsticks with their SmartThings hub as primary, hopefully they will have more to add. 