Lots of people do something similar, but not with the GE master switch. It’s going to be both easier and less expensive to do it with an auxiliary Z wave switch from one of the brands that is designed for virtual three-way. 
You don’t have to worry about tying off the load and the line because these switches don’t expect to control load anyway. And unlike the GE auxiliary, these brands do have a Z wave radio in the auxiliary. So SmartThings just sees it as another on/off device and it works fine.
We were just talking about this the other day: