Ah, perhaps I misunderstood? I thought that you were talking about duplicating and scaling down your house model to create the interior walls of the house.
If you want your cave to have a separate ambient light applied, all you should need to do is make the cave a separate mesh in Blender, and select it with find() in Panda3D. Though, this may leave visible seams as has been pointed out earlier.
simplepbr has an exposure argument. This is a linear scale on the luminance of the scene that you would increase when entering the cave, and decrease when leaving it. There is no auto-exposure implemented at the moment, so you would need to manually tweak it to find the right amount of exposure for your lighting conditions.