There is no direct way on the device to do this. (Unless it's jailbroken). The media library sits in a sandbox, and no app has permission to write to that sandbox, only read from it.

There are 3rd party apps beyond iTunes that can sync to the iPhone. I don't have personal experience with any of them though.

Or an alternative is the iTunes Match service. Any file in iTunes on the PC is then also available on the iPhone without direct syncing.