TL;DR. In the UK, you cannot connect the ADT alarm panel with any other devices or services that aren’t ADT-certified.
Z-Wave Secondary Controller
This is the first solution that I tried. Now I no longer say it doesn’t work in other regions than the UK, but I also can’t verify that myself. Full disclaimer that my experience is limited to the UK market, I managed to put my SmartThings Hub v3 Z-Wave into the learner mode, and was able to use the ADT mobile app to add it as a secondary controller (IIRC, it asked for a code, and I just gave 2222 which seems to be the generic installer’s code that ADT engineers use). Afterwards, I have 5 Z-Wave “Dimmer Switches” shown in the SmartThings app (I have 4 motion sensors, 1 door/window sensor, which I assume that’s why I have 5 Z-Wave devices shown up), but none of them would transmit any data. Later on, they went into the state of “disconnected” perpetually.
To make things worse, the ADT panel would then display a warning saying my SmartThings Hub is an unknown device in the “Smart Devices” section. I was also not able to remove it myself using neither the panel nor the mobile app.
In the end, over a phone call with an ADT technician, they were able to remotely remove my SmartThings Hub from the devices. Now my SmartThings Hub is just in a broken state as a secondary Z-Wave controller. Of course, I could reset my SmartThings Hub but then I’d also need to reconnect my 15 ZigBee devices. Instead of going through that hassle, I managed to use the SmartThings web app to remove these zombie Z-Wave dimmer switches, which is enough to hide them from the SmartThings app.
Qolsys Gateway
I then checked out https://github.com/XaF/qolsysgw which seemed to be the only working integration according to some HomeAssistant posts. Unfortunately, I’m not able to see “Installation” in the Advanced Settings in my Qolsys IQ Panel 4.
Over a phone call with an ADT technician, I’ve been told that menu is reserved for ADT engineers only, and that connecting to other systems such as Google Home, Alexa is not allowed in the UK (but he admitted it is something that customers can do in the US for example). This might explain why some people is able to use Z-Wave secondary controller, yet I’m not able to (though the error is much less obvious).
I do feel a bit conflicted on their approach to this matter, as this effectively breaks my smart home setup into two disconnected worlds: one with SmartThings and HomeAssistant that’s relatively well connected, and one with just ADT alarm system. Hopefully they’ll update their policies in the UK in the coming years, and we could use more competition in this space and someone should provide a solution that integrates with other smart home systems in the UK.