one steam pipe building cant get steam coil for HVAC

indirestriats

I have a 4ton vertical York HVAC system with a compatible steam coil. the problem is that the steam coil requires a return with a pump to the bldgs boiler since it is a one pipe system. The bldg will not allow me to connect to their boiler. How do I find a steam coil that is compatible with a one pipe bldg? I can not attach the return to the same steam pipe because it will cause banging in the pipes. I was recommended to buy a cast iron radiator and mount it on a stand in the location where the steam coil would have been and use that as the source of heat. I was offered to buy an electric coil but so not ant to pay electric charge for heat since I pay for the steam heat in the bldg. this si a 850 square feet office space with 6 rooms. ANY ADVICE?? PLease!!