After wondering why videos I imported into my iPad via iTunes weren't editable in iMovie (iPad version), I decided to do some research into the matter.
Only video that is encoded as 720p (30 fps) can be edited in iMovie. So a way to ensure that video you import to your iPad will be editable in iMovies, do the following.
First, open iMovie on your Mac.
Next add the video you want to be editable in iMovie on iPad. Then in the "Share" menu, select "Export Movie" and choose "HD 720p" as your export option.
Finally open iPhoto and import your videos in it. After doing a sync in iTunes, open iMovie on your iPad and you'll find the videos you just imported!
Happy editing!