I've added an Open Therm GateWay to my home setup. This device is situated between the central heating and the thermostat, and it monitors the communication between those two. Ypu are then able to indert extra messages. The device I bought from Nodo Shop. It's an DIY project, and I had only to solder a few parts. I added an NodeMCU to act as an bridge from serial to WiFi.
I hooked this up, added the gateway to Domoticz and it worked.