While most organizations use Active Directory as their directory service, very few maximize the use of it. Several applications out there like Microsoft Exchange extend the default schema to track Exchange-related information. You can, however, extend the schema yourself by opening the Active Directory Schema snap-in from the Microsoft Management Console. Unfortunately, it is not available by default. You need to register the schmmgmt.dll using the RegSvr32.exe utility (and I thought I would never have to use this utility again). Open a command prompt and run the command below

regsvr32.exe schmmgmt.dll

After that, you can now open up the Microsoft Management Console and add the Active Directory Schema snap-in. You can now add new attributes to the objects as you wish, although, updating the existing ones is definitely not recommended