Extra missions made by the WCS team? That's unlikely, as it requires
a lot of effort and the team said that they wouldn't go through all that again.
Fan made stuff, on the other hand, is more than likely. On top of my head, I can tell that there is already 3 mods being worked on (Enigma, Hostile Frontier and a Kilrathi mod).
Extra missions are relatively easy to do, the Freespace editor is a powerful tool that people have been working with and worked on for the past 13 years. Well, the WCS editor might lack some of the more cutting edge features that have been implemented in the past two years or so, at least until the WCS team releases the source code. That would allow coders to port stuff back and forth between the Saga branch of the code and the Freespace Open's main branch.
If you want basic information about the editor, I suggest you read the freddocs located (iirc) in "<WCS>/data/freddocs". I you require more advanced mission or modding assistance, you can ask on this forum, or on the main Freespace modding community at
Hard Light Productions
. If you don't want to create another account on the forums, you can hop on one of the IRC channels (on the ribbon on top of the forum page, select HLP->IRC->MibbitLinks-> #Freespace or #Hard-light).
If you have any more question, feel free to ask
(I'm not sure my eplainations have been very clear, I'm a bit tired today).