PART I
Start by using this trick on your door
https://tomeofpreach.wordpress.com/2012/11/18/mid-map-entity-spawning/
The door will start off invisible and non-solid, preventing the player shooting it to open it. When you trigger it, the door spawns, and you will be able to shoot it. *
PART II
The big problem now is that until you spawn it, the door isn't there at all! This seems like a big problem, but it's easy to fix. Simply make a copy of the door, and turn it into a func_wall. When you spawn the proper shootable door, killtarget the func_wall version at the same time. The player will never notice the swap!
*Important final hint. In order to use the delay spawn trick on a door, you need to set the following keys on the door:
"sounds" "-1"
"noise1" "doors/latch2.wav"
"noise2" "doors/winch2.wav"
"noise3" "doors/drclos4.wav"
Otherwise you will get a crash when you try to spawn it.