Well, you can ask Microsoft. Good luck with that, let me know how that turns
out for you.
Basically, WordArt is a shape control with different properties. You cannot
"add" WordArt shapes on your own, that I am aware of. However, you can
create scripts that automatically modify a WordArt shape, and you can thus
"pseudo-design" your own specific WordArt shape. The result is the same, you
get a self-made WordArt shape.
HTH
Adrian