adding a radiator to a glow worm economy system?

I am wanting to add a radiator to my central heating system, which is powered by a glow worm economy boiler. Could any one tell me if the boiler tops up the system automatically as you fill the new radiator with water, as I can find no where to top the system up, ie there does not appear to be any small taps or valves to add water to the system. Many thanks for any help.