Tomo_PL: Does anybody know if the F3 GOTY, Oblivion GOTY or Morrowind GOTY are capable of using additional mods? (of course I'm talking about the Steam version). It's the only thing that stops me from buying those games.
Yes.
The only one with any problems at all is Morrowind which cannot use any of the script extenders without a crack. Oblivion has a special version, and I expect Fallout 3 does as well.
Beyond that, every mod works perfectly. And considering the very small number of mods that actually use any of the script extenders, it is likely you wouldn't use those anyway (I stopped long ago since they kill stability).