Transitions in ductwork are inserted automatically when the size of the ductwork changes. Typically this will happen at branches.
If you need a transition in the middle of a straight section of duct, use the “DM HVAC->Duct Centerlines->Break Duct” command to insert a break in the duct. Change the size of the ductwork on one side of the break or the other. When you redraw the ductwork, the transition between the two sizes will be inserted automatically.