You can not return to Broken Valley and the fjords after entering the Hall of Echoes, but depending on the set you may be able to find or buy pieces in Aleroth. In D2:ED certain flying fortresses had complete sets in chests throughout, but they were moved around in DKS and a bunch of new sets added, and I don't think anyone has compiled a list of where they all are yet, or which can show up at merchants.