Sync music from iPod classic to iTunes?
Not officially.  Syncing is intended to go in one direction:  from iTunes to iPod.  

That said, there are some third-party apps that will permit this.

https://tunesgo.wondershare.com/music-transfer/transfer-music-from-ipod-touch-to-computer.html

Good discussion on Apple Support:
https://discussions.apple.com/docs/DOC-3991