This script will create AppleScript droplets that perform a function similar to iTunes 9's "Automatically Add to iTunes" folder but for individual playlists. Simply select a playlist and run the script to create a droplet that references the selected playlist. Once such a droplet is created, drag-and-dropping files to its icon in the Finder will add the files to your iTunes library and copy the new tracks directly to the playlist that the droplet references. The script will allow you to create as many droplets for as may playlists as you like.
Also see Drop to Playlist which works similarly but only for a single preferences-set playlist.