Move the walls to the dormer.
Revit roof dormer.
After adding a dormer cut an opening for it through the roof.
Create the dormer roof and join it to the main roof.
Select your flat roof press the modify sub elements button that appears.
Use the join roof tool to join the dormer roof to the primary roof.
To use this do the following.
Create the walls and roof elements that make up the dormer.
Add point or split line if better suited creates points at intersections place your new points as required.
Create the roof opening and finish the dormer.
Use the modify sub elements button to select your newly placed points.
This video demonstrates the following.