Yes, you can export AD users using any number of tools and import them into CRM, this is not like any contact import job (it's not a built-in feature).
Be sure to not have duplicate your CRM user's email addressses in your Contacts list. - It causes problems in a number of areas if the address associated with a CRM user is duplicated in the CRM Contact/Lead/company record.
Scott Sewell, CustomerEffective | http:\\blog.CustomerEffective.com | Twitter:@ScottSewell