@Chris, the domainsList.xml has been moved to the domains.json file.  You should be able to merge those files into one.  Alternatively, depending on the number of domains you're trying to move over, you can use the "Attach Domain" option.  Doing this does not require the server to be stopped.  This can also be "automated" with a script by calling the "settings/sysadmin/attach-domain" API call.  Here is a link to that API documentation: http://mail.smartertools.com/Documentation/api#/reference/SmarterMail.Web.Controllers.Api.SystemAdminSettingsController/AttachDomain