Setting custom user data right after register

Im working on an app where a user can signup and add personal information (such as address, date of birth, etc.). I then register them via the email/password auth. Since the app is targeted for the german market I then use custom functions to send confirmation emails. So far so good. I now need to safe the custom date somehow when the user registers. Is there a way to save the custom user data as soon as I register the user and they are not yet confirmed, aka. pending? What is the best practice in this case?