I'm sorry, all of what I said was based off of an assumption that was incorrect.
What I was asking for would be awfully useful for volume dampers and other elements of a duct system that do not split the duct, but still appear in the line. I would argue that this would be huge for system family connectors too.
However, what you are asking for sounds more like purchased dampers for fire and smoke, etc. Which do split the system. And Revit likes that.
If you are able to split your duct at the location that you want the damper to be placed at, and you can access the two connectors created at that split point, you can use this method
Document.NewUnionFitting(Autodesk.Revit.DB.Connector, Autodesk.Revit.DB.Connector)
and Revit should seal up the system with a damper that spans that connection. Although I've never tried this. If that doesn't work, maybe try Document.NewTransitionFitting(Autodesk.Revit.DB.Connector, Autodesk.Revit.DB.Connector).
But I think it'll be the former that will fix you up.