You want to steal planetary population?  There are multiple ways to manipulate populations, ships, and planets in Sins, but nothing that lets you steal population.  You have to settle for stealing income (e.g. Embargo) or stealing ships(e.g. Domination).

Lt. Zurok! "Get the probe its obvious these terrans have something stuck up their asses!"

Then you should go play Destroy All Humans.  Upgradable UFO = Stealing Fun

You better watch out what you wish for though, with Rebellion around the corner.  Word from secret sources in the outer fringe sectors suggests that the Space Ponies are gathering together and building their own Titan, with the full intention of taking Candy Mountain back!  Yet you want cows?  Do you have any idea what those shifty looking Space Cows will do to you if you let your guard down for even an instant?  What if they band together with the Space Ponies?  The races in Sins would be doomed!  Doomed I tell you!  That or enslaved and forced for farm Candy on  the Mountain to serve to our new Pony Overlords.