Its not impossible, I have a work around (sorta) but it uses a disguise technique.
this is how it works, you build your map and add a skybox.
Break the skybox (or remove a side) so that the map would leak.
Then, outside the map, build you hidden area (make it close to the skybox and make it so all the walls are solid and cannot be seen trough except one (ill explain in a sec)
So what you have is a box (made witha sky texture on the inside, caulk outside) and attatched to the side of that a hidden room. make all sides of the hidden room solid except 1 (the window side). this is because if the player can see outside the map he will get the HoM (hall of mirrors) effect.
Now to make it so you map does not leak, make another box going around everything (no breaks) and use caulk texture.
you have now got a room outside of the sky
