You can export an asset in an existing scene as a reference. This creates a child file containing the selected nodes and automatically creates the appropriate file referencing relationship between the two files. For more information see Export objects as a referenced file.

To export an asset in a scene as a reference

1. Select the asset(s) you want to export.
2. Select File > Export Selection > .

   The Export Selection Options window appears.

3. Turn Keep only a reference on.
4. You can also turn on options in the Include Options section to include additional animation, shaders, etc that exist outside of the asset. Anything encapsulated by the asset is included automatically.
5. Select Export Selection.

   A file browser appears.

6. Select a location and enter a name for the reference file.

Maya exports all the selected nodes to an external file. It then creates a reference from the asset in that external file to the parent scene.