Hair can only be added to bald areas.
In a few years, as moe hair is lost, the original bald areas may be bigger, but the area of transplanted hair will be the same size. This can lead to a bald zone around the area of transplanted hair. If you don't like that look (who would!) the new bald strip between the hair not yet lost and the transplanted hair will have to be filled in with new grafts.