root is not considered a cPanel account (nor can it be setup as one) and therefore cannot be tracked/limited like a regular account. You can get an idea of your total server bandwidth usage in the WHM under Main >> Account Information >> View Bandwidth Usage, however this is not an aggregate usage for all server traffic.
If you're on a metered port your host should have tools for keeping an eye on your server's bandwidth that I would imagine will be much more useful than trying to hack something into cPanel.
