[dogscoff]

I don't think you can. It would make sense, but it doesn't make much sense that a population suddenly changes its racial characteristics (combat skills, production skills) just because they have been conquered by another race.

The thing to do is to establish a colonise & trade agreement with a friendly human neighbour: Say I'm gas/ oxy and my ally is ice/ methane - I would colonise all the gas planets in his space, and then trade them for all the ice planets which he has colonised in mine.

Alternatively, allow your enemies (only AIs would be stupid enough) to colonise in your space and then take the planets over with troops or intel.