Hello 
community, and particularly the Revit plugin and Model Editor users. We have a piece of good news. The ability to add plenums is finally here! 
Starting from Revit 2.138.4 the Model Editor supports the functionality for adding ceiling or roof plenums to your rooms. Here is the recording that gives you an overview of the feature.
How it works
Set the ceiling plenum depth or floor plenum depth in room properties
Each room now has a property to set up Ceiling plenum depth and Floor plenum depth. By default, the value is set to 0.
QA/QC
By default, changing the properties does not create new plenum rooms in the model editor. This is by design to keep the number of rooms as small as possible. But, when you export the model from Model Editor, these plenum depth properties are converted to a separate room.
There are 3 ways to check the plenums before exporting the model:
Use Alt + P to highlight the rooms with plenums.
Use Color-by to color the rooms by plenum depth.
Use the 3D preview to visualize them in 3D.
Adding plenums as rooms inside the Model Editor
If for any reason, you need to convert the plenums to rooms inside the model editor, you can use the Separate Plenums button. The button is located on top of the Levels table. Using this button creates new levels with plenums as separate rooms. It also resets the plenum depth property to 0 in the host rooms.
One more thing
The plenums are exported to IES VE GEM files as voids. Thanks to @crduggin for the suggestion.
Credits
Special thanks to @franciscocontreras and @crduggin for helping us with scoping the feature on this topic. 
As always, we are looking forward to hearing your feedback! Let us know if you have any questions or comments.
Enjoy and obey the laws of thermodynamics.