As above, email Larian (support@larian.com) with your current save and they can put you back inside the building.
The latest Xbox update enables USB keys/drives in the system settings section, so saved game files can be copied (I haven't tried doing so yet), and presumably Larian would be able to fix them, as well.
The next patch should fix this, I assume, or at least prevent it from happening, but I wouldn't expect it much before the add-on is out (August/September). With additional changes being made to the game, it doesn't really make sense to release multiple patches when a great deal of time and effort can be saved by combining the testing and release procedures for the patch and add-on. In particular, Microsoft has limits on the size and number of patches that can be released for Xbox games.
