Advise On Weight Gain For Prospective Non-Binary Patient with Lean Build?

Hello, I am a non-binary person, assigned Male at birth, and I would like to have a BBL for gender affirming purposes to give me a more feminine shape and some extra volume. I have a rather lean build due to the gym, and I understand that will likely make it difficult for me to get a BBL. I understand for some patients, surgeons can recommend weight gain, but it cannot be rapid weight gain. I am doing my research in advance to understand my options, but I can't find any information about how these patients are supposed to gain weight. What is the recommendation? Gain weight through caloric surplus over a number of months? I would really appreciate the insight as I continue to learn about the options. I have ruled out implants for personal reasons, and that leaves me learning about Sculptra and a Lipo360 BBL.