You already can use any pop living in an empire you have an immigration treaty with to build a colony ship, you have to select a shipyard to build it it won't show up if you use the colonize button on the planet you inted to settle.
Not what I was asking, from the patch notes.
.Habitability map icon now considers all possible species instead of just owned ones
So wanted to know if that means, we can built a colony ship that includes species from empires we have treaties with, that might not currently be in our empire. This would be really useful and make migration treaties more useful early on, since it's easy to get a treaty with an empire that has the complete opposite planet preference from your founding species, thus never seeing a population from them migrate over until you either invest heavily in habitability improvements, get something that has universal habitability up (ring world, ecumenopolis, habitat or Gaia world) or you opt to take the hit and colonize a crappy world just to prompt some of them to migrate over.
If that isn't the case, not quite sure what that change is suppose to facilitate.