I just noticed that the Default Ticket Template already includes the customers email address, so unless you have changed the template settings, within a ticket you should be able to just click on the Custom Fields tab and see an easily copiable email address there already.
Otherwise, to change or add custom templates you do:-
On SmarterTrack v12 (as they moved things around in last few releases so you may need to look around if not on 12).
Change/add templates, go, Settings, Configuration, Custom Fields. Then Templates tab shows all templates, you can edit existing, or create a new one. You can add existing Fields or create additional ones in the Fields tab and add them in.
To change template if you create a new one.
Settings, Manage, Department and select a department. Go to Tickets tab, here you will see what Custom Field Template is being used, probably Default Ticket Template.
We use these when we want to capture certain data from customers and save it in a way that we can pull the data out to put into other systems etc, i.e. Invoice numbers, certain date information.