Yes, it is possible to build a sink over the radiator. A custom cabinet can also include a passage for air to enter under the radiator and come up a chennel at the back wall. You can also remove the radiator and install small ones at the side walls.
An informed installer can do a heat loss calculation on the building and estimate how much radiation you already have to see if you can add heat to the upper floors without buying a larger boiler. Many boilers were installed in the past that were much larger than necessary – which would leave room for more radiation elsewhere. There is a heat loss calculation program to do estimates and a design course at: http://www.hydronic.net