If you want it just to be added into your server, it's quite easy.
Just open your
src/map/map.h and look for all the race declarations. They are on an enum like this on
line 257:
enum { RC_FORMLESS=0, RC_UNDEAD, RC_BRUTE, RC_PLANT, RC_INSECT, RC_FISH, RC_DEMON, RC_DEMIHUMAN, RC_ANGEL, RC_DRAGON, RC_BOSS, RC_NONBOSS, RC_NONDEMIHUMAN, RC_MAX};
Just add your entry there wherever you want, before RC_MAX. It's common practice to add it after the last race that has been declared (RC_NONDEMIHUMAN) but it's up to you. Keep in mind this enum assigns consecutive number identifiers for each item on it. You may also want to add your new race to the scripting constants so that you can use it in scripting in
db/const.txt (look again for the race definitions and follow the structure, but they aren't on an enum this time).
You could also want to edit some atcommands or script commands to adapt them to the new race (especially @mi). Any special feature you want for the race other than existing and being applied to mobs and whatever other non general feature must be added by you.