Apple's tutorial Creating Video for iPod requires Quicktime 7 Pro, but the free Quicktime Player is equally capable.
I've posted both an AppleScript (Export front movie to iTunes.scpt) and an automator workflow (Export selected movies to iTunes.workflow) to get you started.
The Automator workflow enables you right-click on movie files in the Finder to do this, and the AppleScript lets you export the frontmost movie via the scripts menu. One interesting note is that these both work for any media files that Quicktime Player can open.
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