What do you mean by converting cubic feet into square feet? They both are not comparable. 3d and 2d can't be compared. Because cubic feet is the unit of volume and square feet is the unit of area.

You can calculate the surface area of 3d object. Yes!. But you can't convert it into 2d. E.g. You have a cube whose volume is 1 cubic feet. Now you want to calculate the surface area, you will add area of all the 6 sides of the cube. Which will be 6 square feet. Similarly, you can do this with any object whether it be square, sphere, pyramid, etc.

You can calculate the surface area of 3d object. Yes!. But you can't convert it into 2d. E.g. You have a cube whose volume is 1 cubic feet. Now you want to calculate the surface area, you will add area of all the 6 sides of the cube. Which will be 6 square feet. Similarly, you can do this with any object whether it be square, sphere, pyramid, etc.