Can You Have a Fat Donor for Butt or Breast Augmentation?

Fat transfer is essentially a transplant of fat tissue. No one performs immunosuppression for fat grafting, therefore it is not acceptable to use a donor unless that donor is an identical twin. At some point in the future, they are experimenting with taking fat stem cells and making them grow.

The only options for having buttock fat transfer right now are using your own fat. Thoretically, you could also accept fat from an identical twin.
Eventually, we will have te ability to harvest stem cells from a tiny sample of your own fat and grow this into any tissue (including fat) in the body although this may be a decade away,
A for transferring fat into your breast, the consensus now is that the technique may interfere with cancer detection, and it is NOT recommended.