How do I add extra radiators to an existing central heating system?

I need to add two extra radiators [down stairs] to the heating system. I have checked the boilers btu output this is ok.At present all pipe work [10mm] to each radiator [down stairs] comes from manifolds connected to 18mm pipes close to the storage tanks and pump which is up stairs
Is it possible to connect these new radiators[10mm] pipes to the 18mm pipes close to the boiler which is downstairs or should they be conected to pipewok by the c/h pump

Ray Benham
December 2005