You can create a colony on a planet with 0 population. It is essentially a place holder. THEN unload them.
The problem with this is that he can only do that with his home species. The only way I can think that this will work is if he DB edits in a population with the correct raceID and speciesID.
Otherwise what you can do is conquer a population owned by the NPR that species belongs to, once you have a colony with the correct species on the top right corner of the system summary screen the drop down that says "human" will have an additional option for whatever species you have.
The problem with aurora is that the game doesn't realize that there is another species in your empire until there is a colony that belongs to that species - which prevents you from properly creating a colony containing that species for you to immediately use in the event of you capturing a colony ship like this. I had a similar situation that happened to me but I had conquered the NPRs homeworld so was able to create a colony for their species.