How do I create zones inside buildings using area portals?

I have a building that's a .dts(or .dif, if the entrance needs a portal). Now I have an indoor environment that fits inside this building. But the building's collision bounds actually block the player from entering the indoor .dif object. Do I need to create a portal on the outside of that building that links to the portal in the indoor environment? How would I do that? Another example might be, what if I wanted to create an interior that is bigger than its exterior? Thanks