Can Default or Built-In email templates be modified?

For example, for the Outgoing email template Default Ticket Greeting, it is coded as follows:

<first_name>, thanks for using the service desk ticketing system. You can check the status of your ticket (<ticket_id>) at

<login_url>

Can additional text be added such as, "Please do not reply to this email directly, to update this ticket, please click on the Add Note link below or email support@mycompany.com directly, reference your Ticket number." (without the quotes)

I tried modifying a template and hit Save, but it's not showing up with the added text.  Or do I need to create a custom template?

Thanks.