Lindsey,
You should be able to create any username at your domain as long as it does not already exist and you have not reached the user limit. You will also need to be a system admin or a domain admin.
If you are trying to create a user for your domain that has a different domain name on it this is not possible unless that domain is a domain alias of your current domain.
please let me knwo if you have any questions.
Thank you
Tony Scholz
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com