The first step is to map the fields in the AD form to the attribute in the SharePoint user profile. This can be done in the configuration of the shared services provider (central administration> shared services> user profiles and properties).
There you add a new attribute and determine which AD field should be mapped to it. You can also determine which fields users are allowed to override (but only in SharePoint, this is only one synchronization), and which of them is read-only.
This blog post shows how to configure SharePoint search to search users by user attribute in a user profile.
I hope this points you in the right direction.
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