I don't believe it's possible to add to the defaults. You can, of course,
always type in anything you want, and this is still probably the easiest
thing to do, but if you have a specific watermark you want to save, you
could select it (View | Header and Footer) and save it as an AutoText entry.
You'd have to open the header pane to insert it, however, and it won't be on
the AutoText menu there unless it was created in the Header style.
