You can approximate this with a in-place duct fitting (re attached .jpg). Unfortunately, you can't tap off of a duct fitting, but if you get the ducts close to it, it may be 'good enough'. I've seen customers take similar approaches due to this limitation. Of course, you lose the 'flow' information through the duct... but at least you can approximate the physical model.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.