You can simply add a new Field to the CMS, by adding it to the $db array and the required input field in the getCMSFields method.
Pretty much like described in Tutorial 2: http://doc.silverstripe.org/doku.php?id=tutorial:2-extending-a-basic-site#adding_date_and_author_fields
These fields can also be accessed in the navigation. The name (eg. MenuSubTitle) is entirely up to you (as long as it doesn't conflict with existing field-names).