As for iTunes, yes it's possible. One thing I found as an issue with iTunes libraries on an external device of any kind was that it took a lot longer for iTunes to find the next song...every time. This was a while ago, but it was on a 100Mbp wired network, so it wasn't the network media giving me problems. Maybe Freenas is faster today than the software I used then, but there's still going to be a different - and longer - delay.
I have no experience with Plex servers, but as long as your NAS storage is big enough, you should be able to make it work as long as Freenas supports Plex.