You can move over the domain folders from 13 to 15 (while the service is off) then modify the domainList.xml file in
(C:\Program Files (x86)\SmarterTools\SmarterMail\Service) and add to that list the domains you transferred. Once that is done you should be able to boot the service back up and you'll have those domains now.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com