Camera Connection kit with USB Key

yes, as long as the files are in a directory named "DCIM" on your USB key, they will be seen by the iPad when you plug it in via the Camera Connection Kit's USB connector. any files outside the DCIM directory are not seen by the iPad. Files in subfolders within DCIM also seem to be found and imported (though the directory hierarchy is lost on iPad side after importing)

so in other words, yes you can manually store images or videos on your USB key, and as long as they're inside DCIM directory the iPad sees them and can import them.