Hi John,
Because of the way Popups work, it's not possible to do this (at least not with the templates at the moment.) Popups are populated onto the page when the page is created (and "appear" and "disappear" when required.) So it would take a lot of hand-code to make them work off a checkbox.
I recommend for now either making the fields on the first form (and hide / unhide them) or do the popup as a button.
cheers
Bruce