Hi M,
Any additional information on this yet? I found this in the forums, and a secondary controller seems to be working for others: ADT Garage Door Opener / ADT Pulse / ST v2. Talks about more than just garage door opener.
I would like to be able to make ST my primary controller and ADT secondary, but I’m not sure that’s possible.
Be interested in hearing what success you’ve had.
Thanks!
B
Chris - I also was able to add my hub to an existing zwave network as a secondary controller and used that to bridge to my amazon echo. All worked fine. Today I added two new devices and can’t see to add them. Did you ever figure it out?
Did you ever figure this out? I’m having the same issue.
I have given up using ST as a secondary controller (and using my ST). It worked fine for a year or so, a recent firmware update messed things up. This firmware update has made ST no longer compliant with Z-wave specifications. I contacted ST support their short response is they don’t support ST as a secondary controller (so non-compliant). I replied that I had no use for the device now and I was junking it. They responded that I shouldn’t do that and they would be happy to help me use it as a primary controller. UGH. they don’t get the point. Anyway I’ve purchased a Homeseer and use that to bridge with Amazon Echo - all works fine.
Thanks! Are you using HomeSeer as your secondary controller? Was it hard to learn? I may do as you did!
Thanks!!!
Just an update: in mid 2017 smartthings added a note to the official support knowledge base that using the smartthings hub as a Z wave secondary is now officially discouraged.
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.
It is more than discourage - it doesn’t work. Devices disappear, become uncontrollable. It is unfortunate.
Robert - actually I don’t use it as a secondary controller. My issue has been that I have a Leviton OmniPro II that pretty much so does everything for me. Lighting schedule, alarm control, door locks (note door locks are difficult due to added security). This works well for me since I can have activities take place based on alarm settings (AWAY, VACATION, etc.). My challenge was integrating with Amazon Echo. That’s when I started down the ST path. I used ST as a secondary controller as it had an Alexa Skill. All worked fine until the firmware change as noted above and discouraging the use as a secondary controller. So I went to HomeSeer which does have a Z-wave plugin, but better yet (for me) it has an OmnIPro II plugin. So I use the Omnipro plugin to control z-wave via the OmniPro. This also helped me with a problem I had with ST in keeping all of the devices synched (that is OmniPro knowing when ST turned something on/off and vice-versa). The good part is that HomeSeer works seamlessly with Echo. There is a z-wave plugin and I do believe it can be a secondary controller. HomeSeer is pretty easy to use/program. I can now do all kinds of fun things like send text messages when my alarm is set to off, or if the temperature in the house gets low (there is a plugin for my Honeywell thermostat). I can also close the garage door when alarm set via a plugin for the garage door opener. My only caution here is the cheap version of the homeseer limits the number of plugins to 5 I think. Think about how many you might want, if more than 5 you may have to go for the more expensive version. Also note, plugins are not cheap 
Hope this helps.
do we know if this is still the case? I dont see the reference on the site, discourging adding ST as a secondary.
Yes, it’s still true.
https://support.smartthings.com/hc/en-us/articles/204392790-Z-Wave-General-Info
When the SmartThings Hub is added into another Z-Wave network, it may not receive notifications from some battery-operated devices. SmartThings Technical Support does not provide assistance to consumers using the Hub with other Z-Wave controllers.
The fact that they won’t provide any tech support if you try it indicates they don’t think it’s going to work very well. But they can’t tell you not to do it because they are a certified zwave controller. So it’s technically possible, but results may not meet expectations. 
Have you had a chance to read the community FAQ?