My upper cheek is sagging after buccal fat removal. Will fat grafting into the buccal area to help support the upper cheeks?

The results of buccal fat removal depends on the cheek bones. Patients with normal high cheek bones always have an excellent result with buccal fat removal. However, when buccal fat is performed in patients with poor cheek bones, the face sags and therefore, the face will have an aged look.In my practice I always cheek the cheek bones in patients interested in buccal fat removal. If the cheek bones are weak (eg. "no cheek bones") the patient must enhance the cheek bones before or at the time of the buccal fat removal. If not, I will refuse performing the surgery because the above mentioned reasons. The link below helps to understand the importance of cheek bones on buccal fat removal.
Patients interested in buccal fat removal should make a consultation with a board certified plastic surgeon specialist in buccal fat removal.

It's difficult to give an answer without photos / seeing you in person.  Fat grafting may be an option.  Also, filler / fat augmentation of the cheekbones may be a great solution to the problem. I recommend discussing the matter with your surgeon.  It is important that you seek the advice of a surgeon who is certified by the American Board of Plastic Surgery and who has extensive experience with buccal fat reduction surgery.  
I wish you the best! 
Dr. Gabbay.

This is difficult to answer without pictures. Both implants and fillers in the cheeks can create good results for patients who have sagging cheeks.  Both can build up the cheeks and help bring proportion to the face.  You must balance the pros and cons of each procedure and do what you feel is the best decision for you.
Fillers:
Pros:
No downtime
Reversible
Quick
Cons:
Temporary
More expensive in long run
Implants
Pros:
Permanent
Great for patients with poor bone structure
Less expensive overtime
Negative:
Risks associated with surgery
Possible infection
Downtime