Log in to the Webadmin interface using the administrative credentials

Open the "Manage Domains" page under "Domains & Accounts" context

Click on "Edit" for the domain you wish to add the storage to

Open the "Storage" tab

Scroll down to "Message Storage:" section and click the "Add Location" button

In the text box of the new message storage enter the path to the new message storage location. This path and filename should respect the pattern set by the previous message storage(s) in regards to path and filenaming.

Click on "Save Configuration"