Can you donate fat so that women without enough can get a BBL?

In theory, yes but you would have to be put on immunosuppressant medications to hope for survival of it. In reality, no because no one would put you on those meds to do so with the potential risks.

Thank you very much for your question.  At this time, that is not something that is done because the body would likely reject the fat from somebody else (like giving a kidney to somebody without medications to prevent rejection).  In the future, this may be a possibility, but not at present.