You can already buy Doom 3 BFE through Steam?
Did i say that? No.

I was talking about buying a DVD retail version and not downloading the game from steam.
My question to you was a general one as you might have experience with that:
Does steam prevent registration of a foreign hardcopy (a british DVD version for example) in your country (Spain)?
I don't have any experience with that but i usually buy my games at Amazon.co.uk in Great Britain and play them in Germany without any problems.
In fact, i own a lot of games, which where never released over here but none of them is steampowered.
There might be the risk that you get problems registering (and therefore running) a foreign DVD copy in your country if it needs to be registered via steam.
If not: Order a foreign version via any internet seller in Europe that has Doom3 BFG in stock.
There are no customs inside the european union and most internet sellers sell games pretty much anywhere you want to.