Fat transfer can be performed after breast reduction and a good option to correct asymmetries. At 5 weeks, it is still a bit too early to make fine decisions about your breast. Discuss this with your surgeon to determine best options.
Without seeing you in person it would be hard to answer. Average fat grafting to buttocks is 400-600 cc. It takes about 300 cc of fat to each breast to increase one breast size. It would take 2.5 to 3 liters of fat out to do all of this. Might need to harvest both from abdomen, hips and thighs....
Fat transfer to breasts is a procedure that is slowly gaining popularity with plastic surgeons. Certain procedures may be more popular in certain areas of the world. You may want to consider a surgeon in the United States if you have not found surgeons in Europe that perform this...