Daniel,
I have not heard of any plans for them to migrate to the W-S/FC System. First of all, they simply do not have enough responses in W-S/FC to support it as their main way of communication. Especially sine the choppers are no longer based in W-S (AirCare 1 is at Lexington Airport and AirCare 2 is at Elkin Airport). WFUBMC really don't get a vote in the matter. The radios that they have/use in the ED are actually maintained under the FCEMS cache of radios. If they wanted to purchase radios and program them with county & city fire/EMS/rescue talkgroups then that would certainly be allowed and wouldn't cost anything but the price of the radios, programming and installation. And I'm sure the powers that be over the system would let them have their own talkgroups for a nominal fee.
It is my understanding that any public safety agency that is privately owned/operated (private ambulance services, air ambulance transport services, hospital based transport services, campus law enforcement agencies, etc) can get on board the system as long as they are a paying customer. I know of at least one private university that is in W-S that uses the system for their primary comms, It would be interesting to know the terms for such an agreement.
We were recently informed that Novant's Critical Care Transport units now have radios programmed on our system, but as far as I know they do not have their own talkgroups, they simply have access to FCEMS talkgroups. The only reason they have this is because they purchased radios for the VIPER Medical Network and decided to add the county stuff in them too.
As far as roaming goes, there is none that I'm aware of. Once you get out of range of the system, you simply don't talk. This is standard across the board for all users of the system.