When you have a rectangle, polygon, or triangle element selected, the Element to Path option becomes available. Selecting this option will convert that shape into a path (a pen element to be specific). You can then modify that in any way that you would modify a path. This option is also available by right clicking a selected shape.

When you have a drawn path selected (pen, marker, etc.), you can change it to any of the other path types by using the Change Path Type option on the Effects tab of the toolbar. This is also available by right clicking a selected path element.