Okay, for the map editor, you need to edit the Definitions file, and in the regions section (right at the top), you can add in new races. MAKE SURE THAT THE NAME HERE MATCHES EXACTLY WHAT THE SHORT NAME IS IN THE COMMONSETTINGS\RACENAMES.gf.
I can't stress the above point enough, if you don't do this, it will not load the map, period, no exceptions. You cannot edit the short names either, but you can edit the long names if you wish.
Once you've made your map, you need to make the two default files compatible.
For the core file, for each race, they MUST have a FF, DD, CL, CA, BCH & DN. Plus a Listening Post, a Weapons Platform, and one of each of the three types of bases. These are minimum requirements, you may need to copy ships for some races.
Now, in the loadout file, you just need to add in the ships as normal, except in the first column, where it says race, you put in the short name of your new race, Cardassian for example.
Then you just have to load the map up and hope everything works out fine. Takes a while to get right, but it's great when it does work. Check this pic out below. (the map below is actually bigger, but this was the best pic I could get with all 9 races on screen at once.)
Quick Colour guide.
Lighter Blue - Federation
Red - Klingon
Green - Romulan
Orange - Borg (or in my Dominion Wars mod, the Dominion)
Brown - Cardassian (or in my dom wars mod, the Borg)
Dark Blue - Pirates
Cyan - Rakellians
Purple - Species8472
Magenta - Ferengi