A prehung door comes “prehung” in a frame. You’d have to either pull out the existing frame to put in the prehung door, or - I’ve never seen it done but it could be possible - leave the existing frame and buying a prehung door sized to fit inside that frame, similar to what’s done with a replacement window. You’d lose width and height, and you’d probably end up spending a bunch extra to get a custom-size door (it would probably be about 78 in. tall, and about 2 in. narrower than your existing door). You could also cut down an off-the-shelf prehung door and frame to fit the space, but then you’re getting into territory similar to installing a slab. Like I said, I’ve never seen anyone do this - never seen anyone want to shrink their doors - but there’s not really a reason it can’t be done.