In La Liga there is an option that teams can transfer players even if the transfer period is out of date. But using this option is either if the player is out of contract or if you have injury player in one position and you can transfer another player in the same position but from clubs in La Liga, and this player cannot be used in international match but only for La Liga matches.
The case now with Barcelona is that after the injury of Alex Vidal that will keep him out of the pitch until the end of the season, Barcelona doesn’t have a quality replace so they are looking to find a fast replacement as a transfer from other La Liga club. They have scouted already players like Mariano of Sevilla, João Cancelo of Valencia and Mario Gaspar at Villarreal but all of the three are key players in their clubs so its hard to believe that these clubs will allow them to transfer in Barcelona.
If they can’t transfer any of this player they will have to find a solutions from the Barca B team, but it’s hard to believe that they could find a quality player that can compete in all three fronts, La Liga, Champions League and Copa del Rey.