[FWB]

funny you mention this i just added another zone to my boiler. find an old column type cast iron radiator. run feed and return, from the heat side of the boiler,not the domestic water side. you will need a circulator which is switched by a thermostat on the wall. there are online sources for schematics on this.
plumbing supply houses will have "monoflow" inline orfices they look like tee's
but have a restrictor in them, this is needed to keep the water flow down so the hot water spends some time in the radiator to heat it up.