ok.. let me see if i understand what you want to do. you want to attach an air stone to the powerhead output, where the water and air (sucked in from the venturi) come out?
it's not possible. it just doesn't work that way. you need an air pump to run an air stone, no matter what the size of the air stone.
i thought of something else, are you pumping air into the Venturi airline on the powerhead? i've never heard of that, but i can see where someone might try it. IF.. that is what you are talking about you MAY be able to put an airline T in and split off enough air pressure to run a stone, but that would be iffy at best, air is going to follow the path of least resistance.