Banana roll deformity. Would fat transfer help?

Great question!  Fat grafting intros area can be very helpful for your problem.  It helps in two different ways, filling in the depressions and supporting the weight of the buttocks to minimize it's effect, which is to put weight on the tissues below accentuating the depression or crease.

Extreme care is necessary when addressing the lower gluteal crease.  There is no one solution to this problem.  Very gentle liposuctioning can help reduce the size of the roll, however, care has to be taken to not obliterate the crease.   Skin tightening procedures such Refirme and ThermiRF can help, but not dramatically.  Fat transfer to the upper buttocks will give better projection and fullness to the buttocks but will not dramatically improve the banana roll.  Have a consultation by an experienced surgeon and have reasonable expectations on the results.  Best of luck. Dr. Michael Omidi.