Adding custom blog_option fields to New Blog Template signup form.

We want a single sign up form (like the one you have listed with Gravity Forms integration) where the site will be created with the New Blog Template (containing the menus, content, theme, and all that) but also contain fields for our theme options (we have created options for city/state/zip/etc).

What would be the best way to go about this? Is there somewhere we can hook into the plugin to add more update_blog_option’s?