Yes. Most definitely. You can have additional fat injected into your buttocks. I have had a couple of patients who were so happy with their results that they came back a year or two later and had it done a second time to get an even larger, more dramatic enhancement.
The timing of when is best to have it repeated is a great question.
If you think you might be happy with the size of your buttocks in general if the top area wasn't so prominent, then you should probably wait at least 6 months, as this area will definitely decrease in size for about the first 3 months, then the remaining fat will likely settle down, or (I hate to use this word) "droop" within your buttocks, causing less fullness up high and more fullness slightly lower. This can take 6 months to a year. Again, I'm not saying that your buttocks is going to droop, just the fullness will likely a little lower in a year.
If you are hoping to be even larger all over, having this procedure repeated earlier would likely provide an even Greater percentage of the fat to survive due to the increased vascularity during the first few months after your first surgery. There really in no "Correct" time to have this repeated if that's what you are looking for.