You can transfer Revit geometry in Autodesk CFD but you will also need one more volume to represent the air. You can create that volume inside the Autodesk CFD if the volume does not have too many edges, but for a little complicate volume I suggest that you use Dynamo script.
The recognition of the volume, which will serve as a base for subtraction could be done in a couple of ways. I did not have too much time to play so I create a quick one. The logic behind the script is that you find your base volume and separate that volume. All other volumes we will unite into one volume and then we will subtract those two volumes.
Please make sure that before you start using this script that you back up your existing project model.
Also, if you want to download this (and all other scripts) with my sample files please visit the Shop page.
The script uses the following Dynamo package: Clockwork v.2.3.0