Without seeing the actually problem I can not comment on the specific feasibility of your specific needs. Fat grafting is commonly done to improve defects throughout the body. When dealing with body areas relatively large amounts of fat need to be assessable.
Provided one has enough fat to harvest and process for injection, fat injections can be done in the legs. How successful it would be depends on how much volume one has to inject, the amount of fat volume injected and how much of the injected fat survives.
Fat transfer to the leg is possible. I suggest seeing an expert as contouring of the fat can be done and transferred to any location to create a new shape and size. You want a VASER 4D HI DEFINITION contouring specialist. Unfortunately cost cannot be determined without further assessment and evaluation. Best, Dr. Emer
It really depends upon what the issue is. Sometimes it can require a series of injections to get the desired results.