How can I add additional connector (glue) points to a shape in word or powerpoint?

You can't add more glue points but you can draw an additional shape with glue points where you want them then group it with the first shape.

[Returning years later for an edit]

Now if you right-click the border of a shape and choose Edit Points, you can add new points to the shape; these become glue points that connector lines and such will snap to.