invasion 101 ???

[General Hawkwing]

Or you can use transports to move the 'slaves'/new citizens to your worlds that have the same atmosphere of the conquered race. Note, leave some space (1M) in the transport incase the planet is full of population. You can not drop new population on a full domed planet even if the new population can breath the atmosphere.

[Tomgs]

Also if you are moving populations around remember that if you keep two or more races on the same world too long they will get unhappy. They don't seem to like being on the same world even if they both breathe the same atmosphere.