The problem is that we cannot get the list of users from Bugzilla. If we could, then spammers would be able to do that also.

So we catch up with the user list as we download bugs. For example, assign a bug to a newly created user and synchronize Deskzilla. You should get the user in Deskzilla. (It still may not be shown in drop-down boxes, because users there are filtered based on frequency of use.)
Anyway, you always can enter user name manually, even if Deskzilla does not know about the user.
Thanks for the good question!
Best regards,
Igor