Can't tell you exactly how to do that since I don't know what torrent app., version of iTunes or even if Windows or Mac.
I hope this will be a guide for anyone new to iTunes and/or Torrents.
In general you would want to:
1) Copy or move the actual files from the torrent to another location on your disk.
Music files would be typically files with names ending in .mp3, .wav, or .fla as well as others.
Video file names typically end with .mpg, .mpeg, .mp4, .mov, wma etc.
You could even put them on a removable drive but if you do that, the removable device MUST be connected or inserted when
you wish to access/play/sync those files in iTunes later.
You may want to create a new folder for each torrent or album for organizational purposes, than copy the files to that folder.
You can usually find where your torrent program stores the files by going to the settings and looking for data storage location.
If the torrent is an archive file with a name ending in .zip or .rar you may need an application to extract the files from the archive.
Note: if you move the media files, they will no longer be available for seeding from your torrent program.
2) Launch your iTunes program.
In recent versions of iTunes, there are some preference settings you may want to verify before adding the new files to your library.
The first has to do with syncing your iTunes program and any iDevices like iPhone or iPod.
Select the devices tab/section and set the "Prevent iPods, iPhones, and iPads from syncing automatically" according to your
requirements. You may not want to automatically sync a device when you plug the device in to your computer.
The following settings are on the advanced tab/section.
Make sure "Copy files to iTunes Media folder when added to library" is unchecked unless you have plenty of disk space available.
If this option is selected, every file you add to your library will be copied ( again ) to the iTunes media folder.
The other option you want to verify is "Keep iTunes Media folder organized".
This setting could re-arrange your display and files based on the song title, disc and track number. ( reference iTunes help )
3) Add the new files to your iTunes Library.
This may vary depending on your iTunes version or system.
If you have the menu bar displayed, you can use: File - Add File to Library...
If you don't have the menu bar then use Ctrl+O ( oh ) ( Mac may use Apple+O, Command+O ? )
and browse to the folder where you copied/moved your media files to.
From the File menu you can add a whole folder of files too. Use: File - Add Folder to Library... and browse to the folder.
I'd just add that if you want to continue seeding, you can copy the files to another folder rather than move them (if you have enough disc space to do so of course). That's what I generally do although, at one stage, I had my torrent downloads saved to an external disc and copied them to the PC as required. That acts as a partial backup too.
Mind you, I avoid iTunes where possible as I'm hopeless with it