There are a number of things you could add (see http://tmib.net/osticket-1802-email-template-variables). If you have attached the forms to Admin panel -> Manage -> Forms
-> Ticket Details you can add it to your email templates by adding:%{ticket.variablename}into
the template that you want the value in. Jared has posted on the forums in the past
saying that you can do the same for variables post in Client details
using %{ticket.user.variablename} but I've never tried it. He also says
that data associated with other forms (such as those added to help
topics) are not currently available. (source). Again I haven't tried either in a while so that may have changed with the last update.