Build a vestibule. That's your option.
But seriously, please do not try to do it. While gluing an armature plate to the door is not a problem, the problem comes with where you actually mount the mag itself, and what it can support. In looking at that door, the only possible way it can lock currently is by a bolt thrown at the bottom, which means the door is secured by a firm surface. The area above the door is not going to be able to support the weight of a mag with an level of holding force, because the glazing is going to give out before the mag will. I just ran into a similar problem with a space that was 100% glass walls and doors, where the walls AND doors went floor to deck, and they wanted 5 card readers. Simply not possible without substantial construction.
If they want access control, either replace that nonsensical door and storefront, or build a vestibule. Anything else is begging for massive trouble.