•
Go into Curve mode 
.
• Move the dot in the curve cursor over one end point of an open Curve Mark and click the left mouse button (as you would when starting a new curve). If the curve is referenced and this is the common shared point among the referenced curves you will not be allowed to do the extension. Otherwise a menu will appear with these options: "Extend curve from selected point", "New curve starting with selected point", and "Cancel".
• If you choose "Extend..." the existing curve will be added to as if you were continuing to draw the existing curve.
• If you choose "New curve..." a new curve will start with a new point (not shared with other curve) that is at same x,y as the clicked on point.
• End the new curve draw or extend as you would a new curve draw.
Note that if the point clicked on to perform the extend is part of two curves the Which Curve Dialog will appear. You pick the curve and that curve will be the one extended.