Make iTunes accept non-native video formats

This certainly isn't the easiest workaround I've ever seen, but if you have QuickTime Pro and some file organization skills, macosxhints.com has a great hint for adding videos to your library that iTunes doesn't necessarily support.

As long as a video will play with QuickTime, (via installed codecs or otherwise) you simply need to open the original movie file in QuickTime Pro to save what's called a "reference movie," then add that to the iTunes library. This is where your file organization skills come in: that reference movie simply acts as a pointer to the original movie file, so you'll still need to keep the original movie(s) around somewhere.

While this isn't an ideal solution, this will allow you to organize movies iTunes doesn't directly support with the rest of your library, applying metadata, ratings and all that other good stuff. Check it out the full hint for detailed instructions as well as a couple extra tips.