How to add a song to my iphone without replacing the ones already there?

I had to start a new iTunes library and I have yet to sync everything from my previous one. I just want to add one song from my iTunes library, but I can't sync or it'll replace everything on my iPhone with that one song. How can I just add it?

First sync the song and then download/sync all the songs back to your device.

Set it to manually manage and then just drag the song to it.