You can start the process online and yes, there is a $36 fee Transfer of Billing Responsibility (ToBR). There will be a credit check done for you, are you sure that you want to do through all this trouble?
You might want to try that Apple store again, there have been may reports of them allowing Retail Authorized Users to upgrade or add a line to the account. It does not hurt to call first.
Knowlege Base Answer Center Info with basic ToBR details:
http://www.wireless.att.com/answer-...le=en_US&_dyncharset=UTF-8&solutionId=KB94735
Info for how your Dad can start the process
http://www.wireless.att.com/answer-...le=en_US&_dyncharset=UTF-8&solutionId=KB95468
Dave