In my situation the radiators were 60 years old and a one pipe system.  Needless to say. . .  I sold them by the pound when I was done setting up the new system.  There was a conversion package available for my steam furnace.  There is not a lot of demand for steam boilers so these companies modify a hot water one to begin with.  That is why there are conversion packages.  It also came with a domestic coil.  If yours is an older boiler, I am not sure if there is a conversion kit for it.  You could contact a heating/plumbing supplier in your area or find a local rep for that brand.