I have no knowledge of stoves.
Could there be something wrong with the radiator? Before you go to the expense of installing a stove, perhaps a checkup from an HVAC company would be a good idea.
Is the radiator as hot as the others when the boiler is running? I don't know anything about steam-fed radiators, but I know that in hot water-fed radiators (as in my home) it's essential to bleed each radiator every couple of years. If you don't, they accumulate air in them which doesn't heat up and you end up with cold radiators, no matter how hard the furnace is working. Don't know if you need to do that with steam (I'd guess not).
There isn't a shut-off valve that's partially closed somewhere, is there?
