The templates in the Addressing folder are used to create views for clients. (i.e. if you use Outlook and go into your Address Book and choose recipients, it uses the mailbox template to create that view.) If that is what you want, then make the changes to your mailbox template (or rename your template to mailbox.)
If you want to make changes to the view that you see when opening a mailbox inside of the Admin program of Exchange, as far as I know you can't. You can add custom fields inside of the Exchange mailbox (Custom Attributes.)
You probably already know this, but... If you want a template for creating new mailboxes (or importing NT accounts into Exchange) create a mailbox with all the information that is identical for each and then when making a new mailbox choose the template box and select duplicate. If importing NT accounts choose the template box as the template.
Hope that is what you were asking about...