Can you pour fresh concrete on old concrete and it stick?

We rented an old building (about 80 years old) and are renovating it for a retail space. When we started demolation we discovered a sag in the concrete that was covered by plywood-- which is inapropriate as you bounce on the sales floor--- so we ripped it up. The sag in the floor is about 22 ft by 30ft and is 6" deep on the wall side and level in the middle of the space. What would be the best way to make the floor level again. We plan to recarpet the area.