If you want to add any custom races to a game that you are playing as a "custom race" you can go into the race file [ Your Race.raceconfigxml ] and edit the race ID number. The game will allow only one of each race in the game, so you will need to decide which race you'd like it to replace and change your race to that ID#.
Example: The dreadlords are race ID#13. In this raceconfigxml the custom race ID is #12. Now the Dreadlords are now a custom race and the same is true for the reverse function. If you change the race ID to #0 (0-11 regular races, 12 custom, 13 Dreadlords) they would be used in the terran slot.
</DataChecksum>
<Race Name="Dread Lords">
<ConfigVersion>27</ConfigVersion>
<RaceID>12</RaceID>
<CivType>5</CivType>
<DisplayName>Dread Lords</DisplayName>
<Alignment>1</Alignment>
<ShortEmpireName>Dread Lords</ShortEmpireName>
<RaceLeader>Lord Dread</RaceLeader>
<Homeworld>Amalda</Homeworld>