When importing tickets, it's important that in the case of existing users that the user information in the template completely matches the user information in Configuration\User Access & Roles. Otherwise your import will be rejected. Name must match UserDisplayname, UserEmail must match E-mail exactly, and Username + UserDomain must match Userdomain\Username, as seen in Configuration\User Access & Roles.
Also do be aware that if you add users with a dummy email address that you may end up with duplicate (with differing emails) users in your helpdesk.