How to add one song at a time to iphone on itunes?

If I synced my WHOLE library onto my iphone it would be way over capacity but I still have enough room on my iphone to put just a couple more albums. How do I do that without it syncing the entire library. If I select 'sync selected playlists, artists etc.' it threatens to remove all the currently existing songs on my iphone. Can I not just drag and drop one or two songs?

Connect the phone and open the Phone Manager. Set to manage music manually.