I think you may already be able to do this via genemodding.
Conquer the enemy and assign chemical bliss. Create a new subspecies with different living standards. When you want, genemod them to the new subspecies.
It's a kludge. But it can kind of work. Still would be better as a planetary decision but I feel like it really has no intent behind the current design. It's there to be there, and would be better with a purpose.