My friend and Microsoft Valuable Professional Adnan Cartwright wrote an article around mounting an Image to Convert Windows Server 2012 R2 Core to Full Installation

PowerShell is proving more and more to be an Administrators right hand in handling many tasks. Converting a Core Installation to a GUI Installation in Windows Server 2012 R2 is one of those task, read on…..

http://fisg-us.blogspot.com/2013/08/mounting-image-to-convert-windows.html