I appreciate the desire to appear more feminine in the hip region (believe me, I do!) but I've done a bit of research in this area. If you are interested in why it is medically unsound, read on, otherwise feel free to skip to the hug at the end of this post.

I'd say it would likely be impossible given the current level of medical knowledge/technique. Not only would you have to be on strong immune-suppressant drugs for the rest of your life to prevent your body from rejecting the new pelvis and killing you, the nerves that run through the pelvic region are incredibly complex and dangerous to mess with, making it extremely likely that you would be paralyzed and feel nothing from the waist down, if you survived the surgery. On top of that, even if everything worked there would be the problem of mating the top of your leg-bones to the new sockets in the pelvis, reattaching ligaments, etc. Any failure in that chain would render the operation useless.
*hug*