So far as I know, this cannot be done directly. It looks like general polygons can’t be part of footprints.
However, building it out of lines is actually a pretty good workaround. I was able to draw an open star with just five line segments (six clicks total!), which seems to convert into an element just fine. With a fatter line width (and about 30 seconds), I was able to fill the shape with lines, making a solid star shape. And that works as an element too. Not half bad, actually. :)
