Well, this is a two sided question..... However, before answering, there is a difference between "Is it possible" and "Has anyone figured it out yet" . The server kit for SFC3 is RIDICULOUSLY non-user friendly. I'll never understand why, but someone made the executive decision to produce a server kit which is downloaded completely seperate from the game, and has virtually no user interface whatsoever. IMO, this severely inhibits the growth and versatility of the Dynaverse. Having spent the last 2 weeks fiddling around with the server kit, my respect for the server admins has increased tenfold due to the complexity of changing any options for an individual server.
Having said that, a couple D3 admins have successfully added the Cardassians and Dominion as playable races. Unfortunately, due to the fact that there are only 4 available slots for human players to choose from, any additional races you will find will be intermingled with one or more of the existing races. Example: Chris Jones has made a fantastic mod, using ships from the Dominion Wars. All the Dominion, Cardassian, and Breen ships are found in the romulan shiplist. All the romulan ships, are subsequently moved into the Klingon shiplist. Other people have made similar mods, and have also used identical methods to add new races.
Using that as an example, to answer your question: Yes, people CAN add races to a D3 server. But at the same time it is an EXTREMELY daunting task whereas it involves some pretty detailed script modifications to the server kit. This task becomes even more compicated when trying to determine racial alliances. At this point in time, I believe that NOONE has figured out how to add a race while setting up every race as an enemy (ie. every race for itself).
If anyone wants to at least try out a new race, I recommend heading over to the "New Worlds Server". I am in no way affiliated with that organization, but the server admin was a tremendous help to me on this particular subject, and he has modded his server to add the Cardassian and Dominion as a playable race. (Of course, all clients will have to download and install his mod before joining.)