This is poswsible, but not with the Designer itself. You need to leverage a custom workflow that is ecextuted before the "BuildingMachine" state to effectively be able to change the hostname. The goal is to modify the custom property that hold the machine name. You woul dstill need a machine prefix and initially the machien would get it's name from the prefeix. Once the request is submitted your custom workflow would go out, retrieve the name from your CMDB and then update the custom property for the name overriding the name assigned by the prefix.
Give me a few days and I will put together a clog post on this scenario.
Thanks,
Sid Smith