Unfortunately, smartthings no longer recommend adding their hub as a secondary to any other Z wave system.
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.
Some people have been able to do it with some success, but the results have been variable. See the following thread:
Just be aware that if you tried this and it doesn’t work, you may have to completely rebuild your SmartThings network from scratch, which is why support tells people not to do it.