If you add the peanut butter directly into the machine it will just swirl right into the ice cream, you won’t have bites/swirl of actual peanut butter in the ice cream, but instead just even more of a peanut butter flavor. You could try spreading the peanut butter on parchment then freezing that, then chopping it while frozen and adding those bits to ice cream churner. If you’re fast enough they won’t melt and will stay frozen and could give you the same affect. (I do like the ice cream after freezing for a few hours though so its scoopable, straight from the machine its more like a soft serve)