You can make SmartThings join existing Z-Wave network as a secondary controller. This function is buried in the mobile app and not easy to find. Go to the Dashboard and tap the menu (three horizontal bars in the upper left corner). This will open location menu (“Home” or whatever you named it). Now tap on the “gears” icon in the upper right corner. This will open configuration page. Scroll down to the “Hubs” section and tap on on the “Home Hub” (or whatever you named it). On the next page, tap “Z-Wave Utilities”, then “Join or Leave Existing Z-Wave Network”. Finally, tap “Join”. Now put your primary controller in inclusion mode and you should be set.