[dogscoff]

Yup, if you build an atmosphere converter on the planet the atmosphere will (eventually) be converted to the preferred atmosphere of the largest population on it.

Can't remember offhand where the atmos converter is in the tech tree...

Of course it's usually a lot easier to just steal someone else's population and put them to work. Troops are a good way to get alien populations, as are ship captures via boarding parties or alliegence subverters (steal colony ships & pop transports), crew insurrections (ditto) and Puppet Political Parties.

Of course not everyone wants a bunch of dirty aliens running around in their empire...