No easy way. The transfer is one way...from PC to iPod only. Keeps from pirating and distributing. Look for an iPod hack that can do that. Google it, there are plenty. But there is no guarantee that it will not mess up your iPod.
Next time, back up all your music files to a removable disk.
Plug in the iPod, go to it through My Computer, show hidden folders (Tools, Folder Options, the View tab, "Show hidden files and folders"), iPod_Control, Music, and then start copying and pasting the songs from the folders to your My Music folder. Once you have them all in there, set your iTunes preferences to "Organize my iTunes music folder" and then open the songs (or drag them from the My Music folder to the iTunes window). The names should change from things like "ABCD.mp3" and "WXYZ.mp3" to their regular names.
ps. ive never tried this, but apparently it works.