Of course you can do it. The first step is to talk to your advisor to see what's involved. I'm not familiar with JHU's policy on courses to take. Pre-med is actually not a specific major, but rather a series of courses you'd need to take in advance of medical school. In fact, I'm certain there are plenty of poly-sci doctors, since one important criteria med schools use is gpa. Investigate what courses you'd need (Bio + lab, Chem + lab, Physics + lab, Calc, etc.) plus other factors to get involved in like research. An advisor should be able to help with specifics. It's certainly doable, but map out your next few semesters to try to fit it all in. Alternatively, you could do a post-bac, but it's probably not necessary.