getting mp3s off an iPod onto a newer computer

I have an iPod Nano (2nd generation) loaded with mp3s that I had in iTunes on my old Mac Mini computer. But I lost that computer (and so I lost all those mp3s on it). Now I have an iMac, and it doesn't have all those songs on it. Can I get the song mp3s off the iPod and into iTunes on my iMac? Or will iTunes want to erase all the content on the iPod before doing any kind of syncing process? If I connect the iPod to my iMac, I can see the iPod in iTunes, and play the songs on the iPod from within iTunes on my iMac. But trying to do the menu command File\Devices\Transfer Purchases from "Gary's iPod Nano - 2nd Gen" does absolutely nothing. And the File\Devices\Sync command is grayed out. I did buy all that music that's on the iPod in the past (on CDs that I imported into iTunes on the old Mac Mini). So I'm not just trying to pirate new songs from someone else's iPod.