Hi,
Don't try to mix your custom html with the auto generated stuff.
Just remove the
<%Dbsql::HTML::generate_add_form%> (or ALL the default html)
from your add_form.html and replace it with your own html.
You should be able to display the username using <%Username%> and the email by using <%Email%>
and you can use hidden fields to add them to your table -
<input type="hidden" name="Username" value="<%Username%>">
<input type="hidden" name="Email" value="<%Email%>">

To find out what tags you have available in your template, put the following in your template and load the page.
<%GT::Template::dump%>

Hope that helps.
Simon.