Manually manage the content on your iPod to avoid erasing the files already saved to the device's memory. While the iPod provides convenient and portable playback, the automatic sync feature of the iTunes-to-iPod connection method can cause frustration. Adjust the settings so you can put new songs on the iPod without erasing the songs already stored to the device.
Plug one end of the connection cable into the port on the iPod, and plug the other end into an open USB port on your computer. ITunes will automatically open once the connection is established. Click "Cancel" in the dialog that opens.
Click the "iPod" entry in the left-hand window and click "Summary."
Click the checkbox for "Manually manage music and videos," then click "Apply." Wait a few seconds for the settings to take place.
Click the "Library" entry under "Source" to display the music files on your computer. Click on a song that you want to add to the iPod and drag it onto the "iPod" entry in the "Devices" list. ITunes will upload the song without erasing anything previously saved to the iPod's storage.