Email Needed for Facebook Users

I have a subscription manager that will allow users to track a post’s comments via e-mail (which has worked great). Now, that I have Facebook Connect, the users do not have an e-mail address in the profile. Is there a way to prompt the user to update their profile on my blog?

Initially, I thought using the post URL for first authorization would work, but you cannot use the Facebook login button. The button makes the most since for returning users and just a quick way for new users to add my blog to their app. I am open to any thoughts…