Additional scenes, NPCs and items can be made or pulled from the main campaign using The Divinity Engine 2, which is available from the tools section in your steam library Steam.
Its a lot more complicated than the DM mode; but hopefully in the coming weeks we'll get more tutorials about how to do stuff in it.
There are a few already up on the wiki. Make sure you start off with the Larian Setup guide if you take a look.
https://docs.larian.game/The_Divinity_Engine_2