I do this all the time (but I use PC not mac and its not a nano but a classic). I use mine on 4 dif computers so mine is set to manually mange the music, not automatically sync
I open the ipod in explorer (making sure all files are showing) I "unhid" the music folder and copy all the files in there to my itunes library..here are the instructions I found and they work great (I dont know how you'd have to modify if you're using a MAC)
A reader points out a straightforward way for Windows users to transfer music from an iPod to your hard drive, without extra software:
1. Connect the iPod to your PC. If iTunes starts syncing (ie erasing) your music automatically, hit the X in the upper right hand corner of iTunes display, to the left of the search box, to stop it.
2. In Control Panel, Portable Media Devices, double-click your iPod.
3. In the Tools menu -> Options, in the View Tab, check "Show hidden files and folders."
4. Navigate to the Music folder. On my 2nd gen 'pod, the full path is
Portable Media Devices\GINA'S IPOD (F
\iPod_Control\Music
5. Select all the music folders, and drag and drop them into a folder on your hard drive, or directly into iTunes.