Loaded question.
Any race can be added to SFC3, but only four at a time on a D-3 are playable. Rakellians have been there from the start, Species8472, Cardi's, the Big 4, etc.
Here is a portion of the race.gf file from a server kit.
Name = "RaceNames"
// These are used whenever race names are needed. They are the ONLY place you must change these names.
// They MUST match the order of the races. Modders, this one's for you.
[FullEmpireNames]
0 = "United Federation of Planets"
1 = "Klingon Empire"
2 = "Romulan Star Empire"
3 = "Borg Collective"
4 = "Species 8472"
5 = "Cardassian Union"
6 = "Ferengi Alliance"
7 = "Rakellians"
8 = "Pirates"
9 = "Contested Sector"
10 = "All Races"
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All have different map colors already part of the updated server kit. Only fur at once are playable, however, unlike EAW/OP -- which sux!!!!
WaterTiger