^^ If the torrent is still open, you can just select more files.. If you have closed it (not displayed on utorrent anymore), you can just open the .torrent file again, and select the files you want, and if you haven't moved the ones you already have downloaded, it will do a "hash check" to make sure they have completed and then will start downloading the others..
(if you don't care about seeding, I would suggest moving the files you already have to a different folder, and avoid the hash check, because simple things like playing an mp3 (some media players write to the ID tag... play count..etc) which changes the file and then the hash check won't show 100%, so it will need to download a bit more for them)