As you have found, conduit and fittings don't currently have the ability to define nominal diameter and actual dimaeter (as is the case with pipe). As the world moves to more 3D model coordination this has become an issue. We are aware of customer needs, and will review this functionality and prioritize accordingly.
Thank you for your comments.
MS
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.