Unity has a means of exporting scenes (or portions of scenes) as .FBX files that Nu can import. Here's the method for exporting them:

1. Have Unity's FBX Exporter package installed.
2. Be in a PBR Unity Project.
3. Put all desired objects in empty root game object.
4. Export said root game object, unchecking Export Unrendered -
5. Delete all .fbx files except the one you exported.
6. Copy over relevant Unity asset folder hierarchy to Nu Assets/{PackageName} folder.
7. Create a StaticModelHierarchyDispatcher entity (or RigidModelHierarchyDispatcher if you want physics responses) and set the StaticModel property to the FBX asset.

If done properly, you'll end up with something like this -

Unfortunately, not everything will come in to Nu exactly as exported, particularly certain light and light probe properties, but this is a rough way to get scenes created in Unity into Nu.