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

Your best option to increase the size of your buttocks without surgery or filler is to gain weight back to the weight you were at when you liked the size. 

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