If you have created a conductor and now want to extend it by adding additional spans, follow these steps:
Select the Conductor tool on the toolbar
Left-click on the target conductor near the end that you wish to extend (you can extend the conductor from either end)
Take care not to click on a pole, as this will create a new conductor
You can now place additional spans that will extend the existing conductor you clicked on
If you make a mistake and clicked on the wrong conductor, just right-click to de-select and try again.