It's been quite a while since I looked at primitives, but if I'm not mistaken, they are all final, or at least, there are no decent overridable methods that would allow you to hack the primitives in this way. As far as I can tell, the primitives are pretty much stuck the way they are.
If you really want to start customizing the shapes, I would suggest that you develop your own primitives classes (they aren't really all that difficult -- they're call primitives for a reason), and then you can do practically anything you want with them.
- Adam