Instead of using ITunes that will synchronize and erase your existing datas off your iOS device(s). Here is a software i recommend.
With this software you can add, replace, make any change you feel is dim fit for your iOS device.
check out the screen shot
You can perform the following operations:
especially the latest version which v2.6
you can find the download link here
For issue free usage make sure you have Itunes installed on your system.