If after you've finished the route you decide to add an additional waypoint, select the route tool, but do not click on the map to create the first waypoint of a new route. Instead, place the route tool pointer on the route line between the two waypoints, when the mouse cursor changes to show a '+' within the triangle, click once to create a new waypoint. Then, to move the new waypoint to the desired location, hold the mouse button down and drag and drop it to the new position.
If you want to fine tune the new waypoint to specific coordinates, right-click on the waypoint, choose Edit Route Waypoints, select the waypoint, and then type in the coordinates. You may also follow the same procedure to change the name or number of any waypoint.
Suppose you wish to create a new route that overlaps or is adjacent to another. Use the route tool to place the first waypoint, but place it away from the other route (otherwise the cursor will attempt to edit the initial route.) Once the first waypoint is placed, click and drag it to the desired position (on or near the other route.) Then, continue adding waypoints to your route.
You can also append any route to another. Right click on the route that you wish to start with and choose Edit Route. In the Edit Routes window, press Append, and then select the route that you wish to add to the end of the first. Note that the new (extended) route will appear over the one that was selected for appending. Thus, you may wish to hide (or delete) the route that was selected.