Building citizens as Yor

Yes, there's a tech you have to research. It's in the manufacturing branch.

Space Elevators -> Planetology -> Xeno Manufacturing

In addition to the first "Upgrade Manufacturing" it unlocks "Build Population" for the Yor

Note that it also takes 1 each Durantium and Promethium to build population

You can also do an executive order to get a colony ship filled with a unit of Yor. Decommissioning colony ships will send the population to an open population slot in one of your core worlds (it favors the closest core world available).