Populating user data with facebook AFTER the user signed up with their email address

Populating user data with facebook AFTER the user signed up with their
email address

I'm sure there is a feasible solution to this problem but I couldn't find
it so far. In my application,I only want to let users with the certain
email account(such as gmail etc.) for now. Once the user verified their
email, I want to create a populate with Facebook button at the bottom of
my view that will only used once, and it will disappear once the person
populates information from it. I am planning to place this button at the
button of the profile view view controller here. I have a login/sign up
implementation using parse.com as backend and after sign in, user directly
goes to this view. If anyone knows the solution to this, I would
appreciate the help. Thank you!