Someone told me that the common add-in they were using to make transition between 2 elements was discontinued in Revit 2018. So I made mine I’ve been surprised about how easy and short it is :
with rpw.db.Transaction("Create transition"): doc.Create.NewTransitionFitting(connector1, connector2)
So the only thing you need is to prompt user to pick 2 elements (targeting desired connectors) and it is exactly what I described in my previous article.