To reach this door you must first gain access to the cellar. Review the achievement "Six Feet Under" to see how to get there.
The key to this second door inside the cellar is found in the embalming room. There is a cabinet on the left when facing the corridor to go to the body fridge room.
In the third drawer, under some scrap of paper, there is an old large key.
This key opens the secret room and unlocks more parts of the story.