Certain maps, mostly community created maps have open areas in buildings which you can populate yourself.
As for the having a dungeon entrance hidden behind a waterfall, you can create something like that within the Divinity Engine, and if you're adept at scripting, you could create it in such a way that it becomes a usable mechanic I believe.
Otherwise, you may just need to do it more in the style of a D&D adventure and get them to look around and tell them that an entrance is hidden behind a waterfall and load the dungeon in yourself. Much as you would any other area/map within the GM mode.
