You'll need to refer to the whoiscart manual on changing their email templates, but you can change DirectAdmin's welcome email. Login as admin, goto the reseller level, then create a new user. On the bottom of the form right above the submit button is a link and checkbox for whether or not you want to send the welcome email. You can click on the link and modify the template to your liking.
(You can also do this without the admin account if you only have reseller level access)