Yes, it is possible... you just need to manually manage the music on the iPod in order to do it.
Follow the instructions here:
http://docs.info.apple.com/article.html?artnum=304708#2
Once you drag the files to the iPod and they're fully copied over, you can delete them from your computer.
Just note, that as the above user said, if you set it to sync music later, anything that is not on your computer in iTunes will be removed from the iPod. So keep it on manual!