When you sync your iPod to your computer, the contents of your iTunes library and the contents of your iPod's library are made identical. If you want to transfer a song from iTunes to your iPod but don't want to alter any of the content that is already on the iPod, you just have to make sure you're performing this syncing process on the same computer you always have.
Connect your iPod to the computer you have always used to sync content. Connecting your iPod to a different computer, even if that machine has iTunes installed, will result in the loss of all the media on your iPod.
Click your iPod's name. From the "Settings" window, select "Manually manage music and videos." Select the button labeled "Apply" from the bottom of the window.
Click "Music." Click on the listing for a song that you want to sync to your iPod and drag it over top of the name of your iPod in the "Devices" menu on screen. This will sync that song to your iPod without erasing the iPod's existing library. Repeat this process for any additional songs you wish to sync.