Can anyone help me reverse-sync from Ipod to computer?

Hello,
I bought a new computer and asked Staples tech support to take all the files off my old computer and put them on an external hard drive, then wipe the old computer so I could give it to charity clean.
I installed Itunes on the new computer, and plugged in the external hard drive only to discover that Staples had only copied "some" of my files. They only copied 1300 of 4700 songs that were in ITunes.
All the songs are still on the IPod, but I don't dare plug it in to the new computer because I'm afraid it will sync to the new, truncated ITunes and I'll have 1300 songs instead fo 4700. Am I right about this? And if so, is there a way to get the 4700 songs that are on the Ipod into the new computer's ITunes? And will it add to what's in ITunes now, or wipe them and start fresh ( I ask this because I've downloaded 200 songs from my emusic monthly subscription since this debacle, so now the computer has 1500 songs)
Thank for your help
George