I doubt you can, iTunes movies are DRM protected. I'm sure if you search around you might find a tool to strip the DRM, but it's not going to be the easiest of things to do.
(DRM stands for Digital Rights Management, but many call it Digital Restrictions Management, because all it does is restrict)
There's an anti-DRM campaign, well, there is many... A big one is
http://www.DefectiveByDesign.org/