Haunt requires adding a room, but it will seal off a floor. What do I do?
The rules for sealing off rooms have this line at the end:


  If all the remaining tiles for a floor would seal off the floor, then the person whose turn it is rearranges the floor until there are more free doorways.


It seems reasonable to infer that this rule would also apply if adding the necessary room for the Haunt also seals off the floor. Thus the floor will need to be rearranged so that the room can be added without sealing off the floor.

The above rule also says that the person whose turn it is should be the one to rearrange the floor, which seems appropriate given that they are the Haunt revealer who is kicking all of this off.