Connect the MP3 player to your PC and see if you can drag the MP3 files off it onto your PC.
If successful, open iTunes and "add to Library" the files you just dragged from the player
Connect the iPod to your computer, iTunes should launch and you should be able to add those files to the iPod.
In cases like this, it is also good to not allow the iPod to Auto Sync. Any files that are not on the PC but are on the iPod will be removed if you do not control the sync.

I'm sure there will be other suggestions to follow

Good luck.