Just a quick post to flag this article. This is really good to have handy. Note that making changes to schema is risky business and should not be taken lightly but when done correctly it can be quite valuable. Another good option is to have an ADAM directory online if you have a high number of custom attributes to store.
Link to My Research Notes and other usefull documentation : Adding Custom Attributes in Active Directory
I agree with Michael. Please donot edit the attributes for your active directory in live environment before testing them on test servers.