Step1: Download the iPhone to PC transfer.
Step2: Change your sync mode to Manually-Sync before connect to your iTunes.
Step3: Run iPhone to PC transfer and transfer all your music you want to add to iTunes to your computer first. This tool will not erase anything on your iPhone or iTunes.
Step4: Add music from your computer to iTunes.
That nice software works with Windows only and costs a bit more than 20, but the OP didn't state any OS preference anyway. Maybe s/he will be able to find this thread again and respond somehow.