Beyonce's Hips and Kardashian's Butt Through Brazilian Butt Lift?

You have a few questions here so let's take one at a time:
1) Is it possible to achieve through surgery?
The two body shapes you are talking about combine all the moves and surgical techniques using in a Brazilian Butt Lift (fat transfer to buttocks). What I mean is that you will need injection specifically to the upper butt (shelf area), middle butt (projection area) to get the larger Kim Kardashian size and the side butt/hips to get the width of Beyonce. This will depend on how much fat you have to take (although we can usually get quite a bit of fat from skinnier girls), and where your butt and hips shape starts at. It is definitely worth consulting with a board certified plastic surgeon who does lots of these cases to see what your options/limits are and how to meet your goals.
2) About 50-70% of the fat injected will "take" provided you do not sit on your butt for a few weeks. Whatever has "taken" after that amount of time is now permanently in that area. This is because the fat cells grow a local blood supply and are now living in that new area. You also get the benefit of removing all the unwanted fat around your midsection and back.
3) How does weight loss effect things?
After surgery, your proportions have changed...... your waist is much smaller and your butt is much bigger. The proportions or ratio of waist to butt are really what those Kim K and Beyonce figures are all about. If you gain or lose weight, your relative proportions and ratio will stay about the same. If you lose weight, your butt will get smaller, but so will your waist....so you keep that sexy new figure. I tell my patients the best way to maintain their results from lipo are to maintain their weight or lose weight. 
Good luck to you on your journey!
Ramsen Azizi M.D.
Double Board Certified Plastic Surgeon
Chicago  

It is possible to augment your hips and buttocks with your own fat if you are an appropriate surgical candidate. Whether you personally are able to achieve these results is based on your body and what the surgeon determines is possible given your body and safety. Once the fat survives, those fat cells will not go away; however, just like all fat cells, they will change with weight gain or weight loss. I would not recommend intentionally losing weight during the BBL postoperative period as you could potentially affect your results. It would potentially be more difficult to lose weight from the buttocks/hips after a BBL as you would have more fat in this area.

The best way to achieve a celebrity-like butt and hips is a Brazilian Butt Lift. A Brazilian Butt Lift is a procedure where excess fat is liposuctioned from different parts of the body and moved to the buttocks. This reshapes and adds volume to the butt while keeping a natural look. Final results are generally seen about 6 months after the procedure.