It's not quite possible yet, but that's a frequently requested feature, and we're most of the way there. The wysiwyg component we're using, http://hallojs.org/, supports defining extra buttons via plugins, and Wagtail provides a hook mechanism for passing in custom JS/CSS to the editor - we're just missing a bit of glue to connect the two.

I'm cautious about overloading the rich text editor with too many features - things like tables, say, would be better handled as an explicit part of the page model / template rather than kept in an opaque blob of HTML - but I can appreciate the need for more formatting options.