You would need to download the map and do that in an a program designed for image editing. Then upload it again in the altered form. On the map layer, you can select the map, hit Z to see a pop up image, and then can right click on that to download the image. I would recommend cutting out small pieces (like a couple grid squares), uploading them and just laying them over that secret door. Then, when/if players find it, you just select that "patch" and in the right click menu you can "Send to back" which will put it behind the map revealing the secret door.