You can upload any scene you create in Unity as a region in Space.
Once your scene is assembled you have two housekeeping tasks;
1. REMOVE MAIN CAMERA
When you create a new scene in Unity it will by default have a light and a camera. You need to delete the camera (by selecting it in the Hierarchy window and clicking delete).
2. ADD LANDING ZONE
You need to define where users arrive when they log in to your region; and what direction they are facing. To do this, once your scene is set up, from the top menu in Unity select Game Object / Create Other / Landmark
This will place a new landmark item in your scene. In the Inspector window, set the type to Landing Zone and tick Spawn Point.
The in scene object is a yellow edged transparent plane which you can position anywhere in your scene. The yellow arrow points in the direction your visitors will face when they arrive.
Be careful to position the landing zone slightly above the terrain; if a tall avatar's feet arrive below the terrain they may fall through.