if you sync you iPod with iTunes, all the music in the iPod will be deleted.
what you need to do is to click on your iPod icon when you connect it to the computer and then check "Manually manage music".
then you need to create a playlist, put all the songs you want to add in your iPod, then under the iPod click on Music, and you will see "Autofill from:" below the window.
select the playlist yu created and click on autofill.
thus the songs will be added without deleting the ones which were already on the iPod.