Should a player identify a feat from an Eberron Campaign book while playing Dark Sun, and that feat is not environment - campaign race - or setting specific, can the player import the feat and use it for their Dark Sun campaign character?
It depends on your GM, but if you're playing in a book-specific setting, probably not
Which books allowed in the game are strictly a DM's call. While many GMs play with "all books allowed" there are a number that choose to restrict books. One of the more common restrictions are on setting specific material, though it is fairly trivial to file off serial-numbers.
Beyond GM fiat and the restrictions of setting (dragonmark feats are a little hard to port to Dark Sun), there are no real restrictions on usable feats.