Not necessarily even that complicated or advanced, hormonal breast growth is commonplace in the modern-day, real world. If you give the right hormones to any human body (yes, even those designated "male" by any standard, be it based on chromosomes, junk, or whatever), you'll almost always see natural breast development. HC is implied to eat (don't remember her putting food into her mouth ever directly being mentioned, but she orders food and seems to like pickles), so I'm guessing lots of her body follows biological processes. So she probably has some sort of black-market hormone pills, or maybe she jailbroke her body to generate the ones she wants.