Blends and Swept Blends will allow you to transfer from a circular profile to a rectangular profile, but you're going to quickly run into some limitations in the family editing environment.
Blends use a single reference plane - you can't angle the bottom plane relative to the top plane.
Swept Blends follow a path - you can draw a spline that sort of approximates the transition in angle but neither the top plane nor the bottom plane will actually be correct.
You might find better success using another geometry environment like Dynamo or Solidworks, then pulling in the geometry and applying connectors afterwards....you won't be able to constrain dimensions, so it's up to you what's more useful to your workflow (flexibility vs fabrication-quality)
The conceptual massing environment has the degree of flexibility you'd want for the angled form, but I'm not aware of the workflow to get that form into the duct fitting category afterwards.