The only way to gain fat in specific areas of the body is to gain fat in general, according to Women's Health. Healthy sources of fat include salmon, olive oil and nuts, details Mayo Clinic.
People gain fat by taking in more calories than they use, explains Yale Scientific. Higher estrogen levels resulting from birth control pills or hormone therapy may increase fat storage in the buttocks, suggests the Daily Mail. Genetics determine where people store fat, which is usually the buttocks and thighs for women, according to Women's Health. Most women cannot easily lose fat in those areas unless they are lactating, reports Scientific American.