You can’t. The Rich Text WYSIWYG is completely separate from the theme fonts and only has the limited list. You could submit something to the Ideas forum asking to add Google fonts to Rich Text but I don’t really see that working since Google fonts needs to be imported into the site. Having it in the theme options is feasible since it’s easier to import one or two fonts on a whole site basis where as importing fonts on a page by page basis could be more difficult and really slow down the site depending on how many different fonts a user ends up using.