You have no chance installing a HDD in an AirPort Extreme. It is missing the Sata connection on the logic board and a special ribbon data connector. Also even thought there is space in the AE you need rubber inserts and a plate to hold the drive in. It uses 'pressure' to keep the drive in and not traditional mounts.
You could create a modification of running a USB controller on a drive internally with a few wires, control board etc but it would be destructive and possibly ugly. Also you still have the issue of securing the drive.