This is very easy to do with automator or an applescript. In automator create a custom workflow, add the action "add songs to playlist". You can then "save as a plugin" of type "folder action", make sure you set the folder you want and enable folder actions!
edit: on another note, remember that itunes does not support many common tv show formats eg .avi files. For video files that itunes refuses to import natively I use a program called VideoDrive