If you mean just a random building you have to have the unit available in your version of RA2, as FA2 reads the game files - And by the way if you do this it wont work on other pc's unless they have the same building.
Well if your new to modding - dont make maps yet! start off by fiddling about with rules and art.ini which you can download from various sites (makes it easier). the ini files control what happens in a game, like cost of buildings and stuff.