You could purchase a surface mounted letter box similar to this one. Simply cut out a hole in the back face of the letterbox near the top that fits around the existing hole in your exterior security door. Then mount the letter box onto the door in a secure manner.
The means of attachment may require a slight modification from the originally designed method intended for the unit that you purchase.This could happen for boxes that mount on keyhole screw slots like shown below that end up getting cut away by making the hole in the back of the box.
For a box of the type shown above, with the removable front panel, it would be straightforward to access the inside of the rear panel to drill suitable additional holes to mount the box. For letter boxes that do not have easy access to the inside you could resort to using some aluminum angle iron along the sides of the box to make suitable flange to allow screwing to the door.