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 in your computer, you can add them to your iTunes library.
NOTE: For previous purchases from the iTunes Store, you can also re-download them at no cost from the iTunes Store Purchased screen. The link to Purchased is on the iTunes Store Home screen, under QUICK LINKS.