Yep, I did exactly that. I passed a class because the prof graded on a curve, but if he hadn’t no one would have passed, so I learned nothing. I went and talked to the prof that was teaching it the next semester, just before classes started, and he said it was fine fine to sit in as long as I didn’t come on any of the test days.

I suggest you go in person to ask, it might be something they’re not supposed to do, so if you ask in some way that leaves a paper trail, they might have to say no just to cover their ass.