Yes, this is possible. To do so, you need to set the "Polyline" edge style template in yEd's "Edge Types" palette section as the default style for your edges. (The "Polyline" template is the first template in said section.)
To set a style template as the default style for the corresponding type of graph element, either double-click the template in the palette section or right-click the template to open its context menu and choose "Use as Default".
