The way I did it was by using my iPod as a flash drive. This is only for if you have another computer in your house with the songs you want on it. If your iPod music library isn't backed up on a computer in your house, then disregard this post. What you need to do is:
1. Hook your iPod up to the computer with your iTunes library in it and once it shows up on your computer open up "My Computer" on a PC, or just click on your iPod icon on a Mac. If you're on a PC find your iPod in "My Computer" and click on it.
2. Create a folder and name it "Music" or whatever you choose. There will be some default folders in there named "notes" and stuff, but just ignore those.
3. Open iTunes
4. When you get into iTunes highlight all the music you want to move. To highlight just click on the first song you want and hold down shift, continue holding shift and scroll down to the last song you want and click on that and everything in between will be highlighted. Once you have it highlighted, right click on the highlighted material and hit 'copy.'
5. Go back to the iPod folder you created and right click on that and hit 'paste.' It'll automatically load everything you highlighted into that folder.
6. Hook your iPod up to your Mac and open iTunes.
7. When you're in iTunes click 'File' and then click 'Import'. This will open your Macintosh HD. Find your iPod on the sidebar and click on it.
8. Find your folder with the music you copied and click on that. Then highlight all of your songs and click import.
If you have enough room on your iPod to store all of your songs in that folder, then you'll only have to do these steps one time. If you don't, then once your music is loaded take the music folder off of your iPod by clicking on it's icon on your desktop and opening it up and dragging the Music folder you created to your trash. Now you can repeat the same steps and get more music. Hope this helps!!