New vertices can be added to existing boundaries using the Add Vertices option:
Select Add Vertices from the toolbar or the Edit sub-menu of the Boundaries menu.
As you move the mouse, a green cross snaps to the nearest point on the nearest boundary, following the cursor. You can use this as a guide to graphically add a new vertex, by clicking the left mouse button when the green cross is at the desired location. NOTE: the coordinates displayed in the Status Bar at the lower right of the screen, are the coordinates of the green cross on a boundary, and NOT the cursor coordinates.
You can also enter the coordinates of a new vertex in the prompt line, however, note the following – the coordinates you enter in the prompt line must be either exactly ON a boundary, or very close to a boundary, in order for a vertex to be added. If the point entered is too far from a boundary, you will see a warning dialog, and be prompted to enter new coordinates. If the point is only slightly off, a vertex will be added at the nearest point on the boundary.
You can add any number of new vertices by repeating Steps 2 or 3. When you are finished, press Enter or right-click and select Done.