I am NOT that knowledgable of the technique, but sounds like you are wanting to “rotoscope”? I shouldn’t even be replying, though I do use iMovie sometimes for simple stuff, because I do not have a concrete answer for you. Seems though, from what little experience I have, you should be able to scan them back in using 720×480, depending on standard definition or high definition production? You would need, of course, to be able to remove the background (I do this in PhotoShop) so your graphics would come out the way you want, overlaying the video below. I simply do not know if iMovie offers this level of editing. Hope someone with better, more experience, will chim in in time to help you.
