To insert a hanger on a duct, go to
Ribbon: DM HVAC->3D-BIM-> 
Insert Hanger
Pulldown Menu: DM HVAC->3D-BIM->Insert Hanger
You will be prompted to select a point on the duct where a hanger is to be inserted.
Select duct or label at location to insert hanger:
A hanger will be inserted at the specified point. The location of other hangers will be adjusted based upon the newly inserted hanger.