With line tools you can modify existing line. If you want to modify polygon you have to change it to line at first by un checking FILL CONTENT in properties.
With this tool you can join two lines in same container. "Click to select one of the two lines you wish to join together. Now click the Join Lines button in the toolbar. The cursor will change to a crosshair, and a new line segment will be drawn between the last point on line one and the position of the cursor. Click on the second line and a new line segment will be drawn, connecting the two lines together.If you want to cancel click VIEW MODE button.