A new mail user settings panel should be available, named "External accounts" just under "Account".
The pane should contain a toolbar with "add/remove/edit" to opereate on the external accounts, and a grid listing the accounts available, showing just a "description" column.
On add/edit (or double-click) a new window/form containing the same data as in "Account" (protocol,server...) will be displayed, with added fields on top for "description" of the accound, displayname and email, then save/cancel buttons where appropriate.
The grid will refresh once saved (new or edit).
This should mantain a specific table mail.external_accounts, to hold the data for each account.
One primary key column will be a sequence for "external_account_id".