Yes you could align the slot of the box up to the existing slot you have on your door, so that when the post is pushed through it is caught in the box and you retrieve your post from the lockable door at the back. However the flap on the front of this unit could make fixing it flush to the door difficult so you may also wish to consider a box such as the
Contemporary Black - Dual Access Steel Post Box which can be fitted in the same manner but does not have the flap covering the aperture.
Phil R