Anson,
We don't have support for doing that natively, but you should be able to add the button to a dialog that you're creating. The next release of JFBConnect actually moves to using jQuery instead of Mootools, which may help a little as well, but again, it still won't be an option.
Our SCLogin module does already have a modal popup view, if that's what you're looking for as well.
I hope that helps explain, but if you have any other questions, just let us know.
Thanks,
Alex