In more modern versions of yEd, there is a manual workaround.
You first connect the two shapes the usual way, letting the edge end go to the middle of your shape. Then, you click on the edge itself. The edge is highlighted and a small black rectangle is shown at the end. You can now click that black rectangle and drag it to where you want it.
The dragging is freehand, it doesn't snap to symbol boundaries or other features of the node. Unlike the two older workarounds from the other answer, it supports positioning the end outside of the symbol, while allowing you to chose the position visually, without having to enter numeric coordinates.
