If 8-9 frames are drawn out, and they are filling them, then yes, you can add a super. I personally would add another brood box, although technically anything they use for brood is a brood box.
It doesn;t matter much in this case, but when talking about such things it is often easier to mention if you are dealing with deeps, mediums or shallows. In this case I am assuming you mean med or shallow honey super of a langstroth design.