Expand Navigation Volume to include the new room.
If you have no navigation volume I'm pretty sure it gets automatically calculated, which means you don't have to do anything with it.
If you have your main room and zombies walk in it, and you have another room not connected to the main room the zombies will not move. (Actually there has to be an exception to this...)
If you have a stairway or a hallway too small and the zombies won't path into the room, then the zombies that spawn in that room won't move either.
In this case the solution is to adjust those things until the zombies can path properly. In the Docs_modtools folder check the scale_standards.pdf
Hope that helps.