If you have in mind registering for a medical school course, taking exams and being graded, and having the course and credits appear on your undergraduate transcript, I believe you will find that impossible. At most, and I think even that unlikely, you might ask a medical school to let you just sit in and listen to lectures. But why?
You will still not exempt any courses at whatever medical school you may eventually attend.
Use college for college courses. If you need a few extra credits, take a literature course, a poetry course, anthropology, history, philosophy, music, sociology, elementary ancient Greek. Any of those will do more to make you an educated person than any head start on medical school basic sciences.