I have Rome Total War (Gold) as a physical disk(s). I wish to purchase the Alexander expansion, GamersGate being the best bet as I've got an account set up with them and everything.

I was wondering if installing a downloaded copy of Alexander onto a non-downloaded (physical) copy of the base game would work or not? I imagine it would (I not so sure about Steam however as it has its own folder, that's why I'm willing to go the GamersGate route as I'm all set up with them anyways) and I'm not all that knowledgeable about this stuff, so thanks for any help in this quick question. Sorry if it's not all that clear but hopefully people will get the gist

I am 90% sure the GamersGate copy will work and 90% sure the Steam copy will not. Not positive though.