DRM on all videos from iTunes blocks their use in any export format. iMovie automatically transfers videos that you pass to it into DV format, and that transfer is not allowed by DRM. So, unfortunately, you can't do it unless...
The only way that I know of to do anything like this is to play the video through a DVD recorder, recording the content, then rip the DVD back onto your computer using a program like Handbrake/MediaFork, or similar. This gets the video into a non-DRM format, though it has the same kind of downside as burning then re-ripping a DRM song from iTunes. You can loose some quality in the process.
Please note, however, that I believe this violates your EULA, and possibly some laws having to do with circumventing DRM. Therefore, I offer this only as a theoretical method, not as a recommendation, by any means. (In point of fact, I have not tried this method, but have used a similar method to convert some of my old video tapes onto DVD, for personal use.)
Hope this helps...
