Thermostat wire question

I recently had a new “communicating” AC/heat pump system put in for a zone in my house — replacing an old single-stage AC. This zone is also heated by radiators (a zone on a hot-water boiler ysystem). Unfortunately, the installer seemed to think we simply weren’t going to use the radiators anymore. He took out the old simple thermostat that used to control both the AC and the hydronic zone and replaced it with a proprietary communicating stat that controls the AC/heat pump. Unfortunately, he used the existinig thermostat cable, and just sliced the cable that ran from the hydronic system zone box. I now have no way to turn on the radiator heat in that zone. The new communicating stat uses only three wires in the thermostat cable (blue, brown, red). There are a ton of others. So my question is, can I connect the two wires i need from the hydronic system zone box to any two free wires in the existing thermostat cable and then take those two wires at the wall and just attach a simple heating-only thermostat next to the communicating one (which apparently can’t handle two heat sources itself)? Or do I need to run a completely new thermostat cable?