You can modify E911 services for a Full Service Line account simply by logging into your Reseller Control Panel.  You will then be able to click Accounts followed by clicking on the applicable Full Service Line account you wish to modify.  You will then be presented with a E911 tab where you can enable and disable E911 service for the applicable account.  Please keep in mind when adding E911 service to an account you will be billed any associated fees.