There's more to being Forsaken than just being undead, though; which is why the various undead mobs around Brill during the quest aren't considered Forsaken, and why Prince Farondis' legion of ghostly Night Elves in Aszuna also aren't Forsaken. Calia's concern in the 9.2.5 quests was that her unique nature (e.g. being an undead being created by the Light) would make it impossible for her to bridge the gap between her and the other Forsaken - that they were two fundamentally different as beings for her to ever be "accepted." Sin'dane laid those worries to rest, more or less, by telling her that her origins via the Light were immaterial insofar as undeath is concerned. She still has to cross the cultural bridge between her and other Forsaken, though; since she doesn't share in their history as victims of the Third War, and additionally bears the name of and a relationship with their most hated adversary. She's got more work to do before she's got universal acceptance among the Forsaken, which is likely going to be part and parcel of her story going forward.
- - - Updated - - -
Already happened, really. The Botani reanimated dead Orcs in Gorgrond via Nature magic in WoD, and Meryl Winterstorm reanimated himself via Arcane magic in the Troll Wars.