they should be connected from the heating flow and into the return if you have a Two pipe system ,if it’s a one pipe you are never going to get good performance from tall rads.
Existing natural gas supply probably won’t cope with two boilers ,run new off LPG bottles.
Probably could be connected to existing ,you would want the new building to have its own control
Run it on its own combi boiler using LPG ,Electric heating .or yo can buy pipes inside insulated ducting, problem with last one is delivery time for hot water .