Create your own Welcome Emails
As a reseller, the default WHM that comes with your account will not send welcome emails to your clients. A separate billing system would be required in order to have welcome emails sent. Hostgator provides licenses for the WHMCS billing system for you to install.
To edit or create your email templates, go to Setup > Email Templates inside of your WHMCS admin panel. If you would like to create a new template for an email that you may send regularly but manually, scroll to the bottom of the Email Templates page and click on Create New Email Template. From here you can add your new email template.
WHMCS provides more information on this topic in their Support Wiki.