It depends on how are you currently getting music onto your iPod.
- If you're using Sync Entire music library, then recently added music should be added at the next Sync
- If you're using Sync selected Playlists, artists, albums and genres, connect the iPod to your iTunes Library, then look on the Device/Settings/Music pane and add the relevant items to the list of thing to be added to the iPod. You will need to click on the Sync (or Apply) button n the lower right corner to make these changes take effect
- If you're using Manually Manage, connect the iPod to your iTunes Library and drag the music to the iPod