Would a cat nurse kittens that didn't belong to her?

Answer

No. Because the kittens need the milk from their mother

Yes, absolutely. If 2 female cats have kittens close to the same age, they will sometimes steal and nurse the kittens. Strong maternal instincts

Actually, it is pretty rare. I once owned 21 cats at the same time and the kitten can usually tell its own mother apart from the other adult females. It's not unheard of though so yes.