You can add additional junction-type MEP elements to a placed straight segment. (Junction-type tools are the Tee, Wye and Take-off.)
Note: The Duct Wye can only be placed onto a straight, horizontal duct segment.
Adjust the element settings, then click on the placed element. The elements will connect correctly. Depending on the geometry of the newly placed element, additional clicks may be necessary to define its placement.
For example, when placing a Duct Tee onto a regular Duct segment, you will click anywhere on the regular Duct’s centerline:
Next, you must choose one of four possible positions for the Tee branch: should it branch off the right, left, top or bottom of the duct? Move the cursor around, then click in one of these four positions to choose the desired geometry.
Right 
Left 
Top 
Bottom 
See also Start Routing by Placing Junction on a Placed MEP Element.