Syncing new music while keeping the old on 30GB

Hi, I am working with an older iPod (it's a 30 GB IPOD Video from 2006), and I finally figured out how to delete old songs I don't want, but I want to sync some songs from my iTunes library to the iPod, without all my old songs getting replaced.

I have tried syncing individual songs, but I get an error message "ITunes could not sync... because the file could not be found." It says this for all my songs even the ones I KNOW have a file on my computer. What can I do?