The Insert Wall Point command will break a wall in a room into two pieces. The two walls can be given different definitions for loads. The break point can be moved to define a new outline for the room.
To add a point to a room outline, go to
Ribbon: DM HVAC->Rooms and Walls-> 
Insert Wall Point
Pulldown Menu: DM HVAC->Building Definition->Walls->Insert Wall Point
You will be prompted to specify the location of the break on the room outline.
Select wall at break point:
A new vertex will be inserted at the selected point.
To move the new wall point, select the room and use grips.