How do I make site Admin to follow some compulsory steps before a admin aprroves a new account?

Hi...I am new to drupal...my problem is that when a visitor creates a new account and it goes to administrator for approval then the admin can set the account of that user to unblocked.

At that time i want the admin not to assign him role and continue further unless the admin enters some field input say a number which is compulsory to be assigned to user e.x. customer internal number.

This field input is visible only to admin and not visible to visitor when filling registration form for new account.

Please help me on this.

Regards,
P