Say I already have a renderer that can support outdoor terrain and environment rendering. How would I go about adding support for interior environments (for example, like World of Warcraft's dungeons) to my game?

I'm interested both in how I should fit the interiors into my content creation process (for example, I thought about leaving holes in the terrain mesh into which I can "paste" the interior dungeon mesh at runtime) and how to render them (it seems like I'd want a different rendering flow other than a blended texture rendering phase that terrain uses).