It already has happened, though. Robb's wife is the obvious example, but there's also Xaro, Pyat Pree, Irri, and Doreah.
Anyway, of course they're going to include Stoneheart. They could hardly cut her out. Although I do worry that her appearance won't have quite the impact that it did in the books, seeing how Catelyn's character was so neutered in the series.