Using purchased music in iMovie

I have purchased songs from the iTunes store to insert as soundtrack into my iMovie movies. Some copy fine, but I cannot get some other songs to "stick" in iMovie (they won't copy into it). I was told by the iTunes folks that some songs have copyright restrictions that prevent their use.

Is there any way to tell before purchasing them, which songs can and cannot be inserted into iMovie?

Better yet, is there any workaround for this problem? The iMovies are strictly for personal use.