You cannot transfer song files from iPod to computer using iTunes (except for songs purchased from the iTunes Store). However, there are third-party methods and utilities that can transfer from iPod to computer. If you do a Google search on something like "ipod transfer," you should get some links.
Once the song files are on your computer's hard drive, you can add them to your iTunes library.
NOTE: You can also re-download previous song purchases from the iTunes Store at no cost, from the iTunes Store Purchased screen.