If you create an antipope, you can wage war against the pope to place your antipope as the recognized pope. You will not personally control this pope, no, but they will have a +100 pressed my claim modifier, and the +5 dynasty modifier. They will add to your dynasty prestige, as the papacy is a king tier title, but only for 1 generation. They will also take any lands they own with them to be part of the papal states.
Oh, so I cannot become Pope and fill the Church's positions with my dynastic members? Ah well, this is fine. I'm sure there will be a mod for it eventually.