iTunes won't move your music from your iPod back into your library, but there are third party programs that will. One is Senuti. The link below will take you to the Senuti download page. It costs $18.99. I've never tried this because I haven't needed it, but it reportedly works as advertised.
Apple does offer iTunes Match -- which will match and backup your entire library in the Cloud (not just your purchases). There's a $24.99 annual fee for this service.