To add a segment to the end of a feeder, go to
Ribbon: DM Electrical->One-Line Diagram-> 
Add Segment to End of Feeder
Pulldown Menu: DM Electrical->One-Line Diagram->Add Segment to End of Feeder
You will be prompted to identify the feeder where a segment is to be added at the end.
Select feeder near end:
Select the feeder near the end where the segment is to be added. A segment will be added to the end of the feeder. The feeder will move slightly to accommodate this new section.