I have a crappy car so I only carry liability on it. I don't think I can add UIM then can I?

I am looking for a policy for myself for individual health coverage. I don't have an employer per se and my school's plan sucks. The plan I have now isn't useful with routine coverage so I'm trying to get a copay plan but now i started worrying about the fact that I commute and the whole 'what if' scenario. Maybe I should just stop looking so hard and just get what I can afford. I need to stop worrying about the 'what if's' for now before I go crazy. Besides the sky isn't falling just yet