Indeed it is not possible.I found a way to manage what I want to do.I will try to explain...I created a few forms for issues (pc failure / lost password / Mail issue...) with variables. I tried as often as possible to use the same name for the variables (If I have to ask the hostname in multiple forms, I will set the name of the variable to "computername")I created a few forms for specific requests (New employee / Employee leaving or "fired") with variables.I created two set of mails template (one for the the issues and another one for the requests)I created one main department (Infra) and two sub-deparments (Request/Issue)For each form, I linked it to a help topic (with the same name) and set the department for this one to the regarding one (for instance, help topic "Mail Issue" will create a ticket in the Infra/issue department and New Employee will create a ticket in the Infra/request department.Then I updated all the "Ticket End-User Email Templates" and "Ticket Agent Email Templates" of the two sets with the variables used in the forms.
By this way, the custom forms appears in the mail.
The inconvenient is that some fields remains blank in the mail if the variables is not available in the forms fill in by the end users. For instance, I have a form for the pcs failures and another one for the printers failures and the field printer is not available in the pcs failures form. So this information does not appear in the mail and I have nothing in front of "Printer".
But finally it works.
I hope that in future releases, it will be possible to add directly the content of the custom forms in the mail sent to the agent or the end-users.
Hope it will help someone.