This article will show you how to create a custom attribute in Active Directory and associate it with the User Class. You must be a Schema Admin to complete these steps. For this example I’ll show you how to add a Favorite Beer attribute. (Why isn’t this in there already!)
Disclaimer: Editing the AD schema should not be taken lightly. Changes can be irreversible. Do this at your own risk!
Read more How to: Create a custom attribute in Active Directory