The problem is that when I create the executable, all sceneries are "baked into" the build.
I don't know a way to load separate new scenes into an existing Unity build (or if it's even possible).
Hi FPV Freerider,
As an experienced Unity Dev it is quite simple to add new scenes. All you need to do is request the level (in unity scene format), and then add it to your own by importing the scene. Unfortunately this doesn't allow you to add it in as dlc. You would need to compile a new executable