Ghost-like entity help

[TheGreatCthulhu]

I'm not sure about doing it the way you originally intended (maybe in a FC, but I'll let the people who have better knowledge of all the customizable features of the game answer that), however, you can "fake" the effect by having a monster spawn in front of a door (at predefined places, and only if the door is closed), and having some particle effects (like, a billowing smoke, or electric bolts and plasma) to cover it up. This way, the monster would appear to be briefly hindered by the blocked door, only to "teleport" inside the room - kind of like the ghosts of Silent Hill 4, who laboriously drag their bodies through a wall before they enter a room.
Who knows, it might even be possible to create a similar "wall corruption" effect, using the setting which makes all the particles aligned in one plane (can't recall exactly what's it called an the moment, but it's the same thing used to make the effect of paper floating on water).