To what affect? You trying to simply select and edge then extrude it like you would in another 3D package? if so I'm not sure you can do exactly that, but the z modeler edge extrude function should get you where you need to go. Have you seen this video ? https://www.youtube.com/watch?v=GLs7A4V6wWo
If you are looking to retopo in ZB then you are limited to placing verts or use the retopology brush. If you want to extend single edges then you have to use software with that feature like Max.