More Info
Note that this functionality is only available in WorkgroupMail v8 and WorkgroupMail v7 Enterprise.
Detail
Yes, this is possible. Once you have installed WorkgroupMail, you can import all other users from the active directory by selecting Edit | Synchronize Directory Service Users. This will display a wizard which will prompt you to select the user accounts to import from the active directory. (Note, those accounts that have already been added will not show up in this list). Any users that you select will be added to WorkgroupMail. You can also select from a list of users that have been deleted from the active directory but who are still users in WorkgroupMail. Any users that you select will be removed from WorkgroupMail. If a user has been manually added to WorkgroupMail then you can right click on the user in the administrator and select Associate with directory service user…. Doing so will display a drop down list of active directory users, from which you can choose the corresponding Windows user account.