Adding extra fields to the affiliate registration form is possible on our Pro plan and up. If you'd like to add additional fields, go to your Account settings and then to affiliate on-boarding. Here you will find the module in which you can add the fields:
Be aware that the fields will be added to the second step of the affiliate sign-up form, only visible after the affiliate hits 'Sign Up Now'.
Note: This section should never be used to collect sensitive information, such as Social Security Numbers (SSN).