I've mentioned this in other posts too, you can control how your body looks if you work hard enough at it.
Just like any form of bodybuilding, you can control where muscle gets built when you bulk. The trick is to cut down to low bodyfat percentage (7-8% and low BMI), then as Vanna mentioned, do the correct exercises to work on your gluts (your ass) as you bulk back up. This is the standard way bodybuilders get their gains, because you can't help but gain fat while bulking and you need to bulk to gain muscle, so then they cut to stay lean.
You can't gain muscle without bulking, you can barely maintain it when cutting (eating below maintenance calories). Once you gain muscle in your gluts it will proportionally push out your fat.
There are herbs that people claim help them gain fat in hips when bulking, like Maca. I've seen good anecdotal results and will try it myself.