No converting from mp3 to mp3 is a poor idea. Search your computer using your computer search engine for one of these songs. See if a copy exists within your iTunes Music Library. If yes, then it should copy across. Use the manual technique of adding to iPod. The only other reason I can think of why the files won't copy across is if they are mp3pro. iPod & iTunes do not recognise mp3pro which was designed for improved performance for very low bit-rate mp3's, such as 64 kbps to 128 kbps. But it doesn't compare very well with other mp3 encoders for higher bit-rates.

Also go to iTunes menu bar, Edit>Preferences & click the advanced tab. Make sure the option to Copy Files to iTunes Music Library folder is ticked.
http://www.apple.com/support/ipod/tutorial/index.html