AVT is a routine from Scott Botting that allows you to add vertices to existing polylines in AutoCAD. Before getting started, note that this routine only works on the older, "heavy" 2D polylines, not the current "lightweight" polylines. Start by loading the LISP file, then type in the command AVT. Select a 2D polyline and pick a point for a new vertex. The new vertex will be connected to the two nearest existing vertices. If you pick a point beyond either end of the polyline, the polyline will not be extended; it will still connect to the two nearest vertices. Press the spacebar when you are done. This command could be improved by expanding it to work with lightweight and even 3D polylines, but if it fits your needs, it does a good job.