I have two heating systems that are independent of each other. One is a 3 zone hydronic system with 3 thermostats controlling a pump controller. The second system is a air to air heat pump without any form of back up heat. When temp's get to low 30's the HP doesn't work so well and it doesn't seem to have an outdoor temp sensor to stop it from working when temps get below freezing. Ideally I'd like to have one main T-stat that controls both systems (and allows use of existing pump zone controller). At a minimum, I'd like to have the HP not try to operate when temps hit 32F. All suggestions welcome.
I also have two independent heating systems so I will answer based on my experience.
It is not clear which of the three zones your "main" thermostat will control, so I will assume as @ThreePhaseEel did that you want the new main thermostat to control the heat pump and a single zone of hydronic heat.
I'll also assume that both the boiler for the hydronic system and the air-to-air HP are triggered in the same way, by establishing continuity across two terminals on their controllers. This is how my oil boiler and electric boiler are both triggered.
You likely already have a 120VAC to 24VAC step-down transformer if your hydronic system uses motorized zone valves. You can use a relay with a 24VAC coil (DPDT or normally-open DPST) which is wired on one pole of the coil to your transformer and the other pole to your main thermostat. The second wire from the thermostat would go to the other pole of the 24VAC power source. This will engage the contacts when the thermostat calls for heat. The two sets of contacts will be used to close the circuit that controls each respective heating system.
You can also put an outdoor temp sensor in-line with the relay for controlling the HP. This way when the temperature reaches a certain point, continuity on the HP control circuit will be broken and only the hydronic system will run, without any change at the thermostat. Depending if the outdoor temperature sensor makes or breaks connection when the temp falls below the set point you may need an additional SPDT relay to do what you want there.
Hopefully this is helpful even with the assumptions I mentioned at the beginning. This doesn't address any cooling function of your HP, but the above could probably be amended to address that as well.