The LNB is located at the focal point of the dish so that is the correct location to locate the antenna. I would doubt that a roof-mount intended for a satellite would aim low enough for the wifi signal without modification.
I ran across this site today and it looks like an interesting project for those interested in extending their WiFi range. This DIY project involves re-purposing a satellite dish as a WiFi antenna.
I haven't tried building one but it looks like an interesting and not too difficult project.