2 years post op BBL, is there a way to restore the fat without having the procedure done again?

Dear Ms Mauaa17, 
Thank you for your story and questions. Unfortunately with out a exam of your tissues I can can only make general comments. I hope find the following helpful.Short answer: to increase Buttock size and shape you will to undergo liposuction and fat injection again. 
The amount of fat that takes depends:
1) the gentle technique of harvest providing the least amount of damage/trauma to the fat cells
2) how the fat is injected
3) the receipent area (good blood supply, lack of previous scars, etc.)
4) postoperative care
5) genetics of the individual 
The process for performing a BBL is the same regardless of the patients weight or fat location:
1) harvest the fat 
2) purify the fat
3) sculpting the buttock:
   a) with precise liposculpting 
   b) with precise fat injection
The final shape of the buttock depends on:
1) your underlying boney pelvic frame 
2) the shape of your buttock muscular anatomy
3) the elasticity (stretchability) of the buttock skin skin
4) the patients desired shape (where fat is injected)
The bottom line: 
1) fat survival as a graft is multifactoral but usually varies between 70-80%.
2) the fat that doesn't survive is broken down, dissolves and is excreted by our body
3) the amount injected fat depends on how much fat there is available to harvgest.
4) the individuals genetic anatomy and elastic properties of the skin will dictate 
and limit attaining certain desired shapes
5) Just importantly the liposuction should be done to "sculpt" other parts of the body to attain a body that is in proportion and attractive. Thus a small amount of fat can go along way or in contrast a large amount may not give you the desird appearance
One can try to gain weight prior to the procedure in hope that the areas you can the weight can be liposuctioned. 
Caveat: all fat that is gained may not be deposited in areas that can be liposuctioned.
I suggest you collect multiple model photos of your desired buttock shape/size and schedule several appointments with experienced Plastic Surgeons who are Certified by the American Board of Plastic Surgery and ideally a member of the American Society for Aesthetic Plastic Surgery (denoting by membership as having met additional criteria and a focus on Cosmetic Plastic Surgery).
My best wishes,
R. A. Hardesty, MD, FACS
Diplomate and Certified by the Am. Bd. of Plastic Surgery
Awardee: RealSelf Top 100
wwwimagineplasticsurgery.com
4646 Brockton Ave
Riverside, Ca 92506
(951) 686-7600

Dear mayaa17,it is hard to tell for sure without an examination. If you are considering a treatment, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Any fat transferred to the buttock has only a certain percentage of survival, and when you lose weight overall, the cells that were transferred to the buttock also give up some volume.  If you have adequate fat to repeat the transfer, then that will improve the percentage of fat that ultimately stays, but if not, then gaining some weight will refill the surviving cells to where they were before you lost weight.