The next patch will be available without needing to buy the add-on. They are just expected to be released around the same time because they were tested together (any changes made to the original game had to be tested, and the add-on needed to be tested by itself, and with saves from the original and updated versions of the main game).
For what it's worth, Lar stated in the latest update about the add-on that when negotiating the publishing deals this time they make sure they would be able to release patches much faster, without the bureaucracy required for the main game (though they don't expect that will be necessary). Hopefully that would include bypassing any unresponsive publishers to make sure all localizations get updated, if needed.
