Hi,
Songs purchased from iTunes before year 2009 are DRM (copyright) protected and cannot be used in iMovie. After 2009 iTunes abandoned the DRM protection and so songs purchased after year 2009 do not carry DRM protection and can be used in iMovie. Repurchasing the same song is one way of cleansing DRM from the song and making it usable in iMovie.
Another way is to subscribe to the iTunes Match program for $24.99, that will allow you to download DRM free copies of your DRM protected songs. If you are interested in that, go to the iTunes website, click on the Store tab at the top of the screen, and scroll all the way down to the bottom to the fine print. Under Features you will see iTunes Match listed.
-- Rich