Step 1:

Create a folder within DLC\ Dist\ Room with Room preset name (PR-0000000...)

In that folder create a folder name as FBX and an empty level name as room preset PR-000000....

Step 2:

Import all room meshes to FBX folder.
Note: materials, textures, and other meshes like trees, and grasses must be stored in this FBX folder,

Step 3:

Drag all room meshes to the level and set the Pivot location to 0,0,0.