I have been a big user of playlists on iTunes for a long time. I use the playlists to be able find things easier. One of my playlists is "Buster Keaton", where I keep all of his wonderful old vintage public domain movies together.
Unfortunately, the December 2012 update of iTunes has made this pretty tricky. But not impossible! The "Add to" Button, of course, doesn't recognize any videos other than those that are purchased, but it still can be done. Here is how:
After you've imported the video into the iTunes library, select it and right-click (control-click for Mac) to open the menu where you will find "Add to Playlist". This opens up a list of all of your playlists, and you just scroll through until you find the one you want.
I just added "The Paleface" to my Buster Keaton playlist, and am now going to sync it to my iPod and go watch it.