converting radiators

I have a chance to get a bunch of old cast iron radiators set up for single pipe steam. I'd like to use them in my house at some point (years from now) for a hot water system yet to be installed (I don't have central heat at the moment, just a woodstove, located in Maine).

Of course to convert them to a two pipe system, I would have to remove one of the plugs in the radiator which has been in place for who knows how many years in a town with not great water.

Will these plugs even come out so that I could convert them into two pipe radiators? What if any tricks could be used for this? Any other thoughts? Could I drill and tap them, since I would only need a 1/2 inch line? Thanks for your ideas.