Import movies already on computer into iTunes
This is from the Apple Support site:


  What video content works with iTunes and mobile devices?
  
  iTunes:
  
  
  Video content purchased from the iTunes Store. 
  QuickTime and MPEG-4 movie files that end in ".mov", ".m4v", or ".mp4" and are playable in QuickTime Player. 
  Video podcasts.
  iTunes Digital Copies. 
  iTunes Store Movie Rentals (Requires iTunes 9 or later).
  


I've had luck converting non-compatible video for iTunes and i devices with Handbrake, a multi platform, open source video converter.