Hello, Jamesson.
My english is not good so I try to understand - you want to add existing vmdks to Win8 vm, but these disks already added to another vm? If so - you can use SCSI bus sharing to use disks on both vms (ussally this option used for Microsoft cluster) or detach disks from 2012SER7ER first (remove only from vm not from datastore, but ensure that there is no I/O on virutal disk) and add them to Win8.