At the moment we can only offer one Extension Kit per home.
If your current Smart Thermostat is wireless, then you could most likely just add the new Smart Radiator Thermostats and have independently controlled zones. Just bear in mind that we always recommend to equip the radiators in the room where the Smart Thermostat is with Smart Radiator Thermostats.
But if your current Smart Thermostat is wired, it could be that we can't offer to properly control the different zones independently as required (because the Smart Radiator Thermostat can tell the Smart Thermostat OR the Extension Kit to call for heating, but it can't tell the Smart Thermostat AND the Extension Kit to call for heating. In case of doubt, please double check with our support about the specifics of your case.