I don't see why you would work the other way around, or what benefit you'd gain. Since I'm curious, can you explain why you want to do it that way?
Also this is more related to Qt than SFML. SFML does create the window and you can retrieve the window handel, so the questions is if you can use Qt on a window of which you only got the handle. So maybe you'd get a better answer in a Qt related forum.