Hello,
just tried to register my good old boxed Medieval 2 boxed version's CD key on steam. Well... Steam accepted the Code. After that I wanted to buy Kingdoms as it was on sale but steam says that I already own that game.
So I started M2 and then magically a pop up screen appeared asking me which game I want to play (m2 early/late campaign, crusades, teutons....).

If you don't have Kingdoms yet this seems to be a good way to obtain it.