How much weight would I need to put on for a bbl?

Without seeing photos, it is hard to say if you are a good candidate at your current weight. I do not believe that people should gain weight
in preparation for a BBL. The fat that is gained often just goes away after the
surgery. This is why people complain that the fat didn't last. When choosing your doctor for this procedure, be sure to find
a Board Certified Plastic Surgeon who specializes in this procedure and
performs at least 6 to 8 per week.

Photos would really help in answering your question better. However, at your current weight for your height I do not think you need to gain any weight.  Consider reposting your question with photographs. 

I do not recommend weight gain, and you very likely can get a dramatic improvement with your current weight.Find
a board certified plastic surgeon who performs hundreds of
Brazilian buttlifts each year. Then look at the plastic surgeon's
website before and after photo galleries to get a sense of who can
deliver the results. Kenneth Hughes, MDLos Angeles, CA