The Sliding Track Accessory can be installed on the pet door after the pet door has been installed in your home. You actually have to install the pet door first, to be able to reach the installation screws.
The Sliding Track Accessory screws to the surface of the pet door frame, from the inside of the house. It is very easy to attach.
If you want to screw the security plate in place at some point in time, just remove the tracks and screw on the plate.