Has anyone heard of pig fat being used for fat transfer?

No, you cannot use other fat. Your body would reject it. If you are too thin, a Brazilian Butt Lift may not be possible. This procedure depends on the amount of fat the patient has to transfer.

You cannot use pig fat or someone else's fat, unless you have an identical twin.  One of the risks involves the immune suppressant meds which could lower your body's ability to right infection after the BBL.

Pig fat transfer is not a viable option. In theory if it worked you'd need to be on anti-rejection drugs (as if you'd had a kidney transplant) for the rest of your life which is something no legitimate surgeon would consider ethical. You'll need to gain some weight if you want a fat transfer procedure, and there are lots of easy ways to do that. Without examining you or seeing photos its difficult to give specific advice so I'd advise you book a consultation with a board certified plastic surgeon near you to see what your options are. Best of luck!