ON a pc unfortunately at present you'll need to manually import them to your iTunes Library (Control+O). Once in your library, ipod should update with the new additions as soon as you plug it in (well that's how it works on a mac anyway.)

See this: http://www.apple.com/support/itunes/...nt102092b.html

By default, iTunes is set to update iPod automatically. When you connect your iPod to your computer, iTunes opens, then transfers all selected songs in your library and playlists to your iPod, and erases songs from iPod that aren't in the iTunes music library. If you add additional songs to your library during a session and want to add them to iPod, choose File > "Update songs on iPod." And also see: http://www.kapsi.de/files/itunes.html (adding songs)