You can use the sketching tools to define the perimeter of a beam system. You can also use these tools to modify a beam system boundary, created using the Pick Supports method.
Structure panel
Beam System.
Beam System panel
Sketch Beam System.
Line to sketch, or click Draw panel
Pick Lines to select existing lines. 
Sketched beam system boundary with lines locked to supports
After specifying the beam system boundary, you can
Mode panel
Finish Edit Mode. 
Floor plan and 3D view of completed structural beam system