If you go Genetic ascension path, you gain access to fancy traits, including Delicious.
So, playing as Devouring swarm, you can do a one-off species change to include this trait to get more food. BUT after every next raid you need to re-do the species modification, since the new pops that you abduct will not have the Delicious trait.
Question: is there a species+government form/traits setup, that would allow me to roleplay as a 'xenovore' race that can 'breed' other pops for food?
Best Answer
Note: This information might be slightly outdated, options below were available in 2.1
You can eat population in two ways: either as a form of extermination (hive-mind devouring swarm only) or as a form of slavery called "Livestock slavery"
Any form of government that allows alien slavery (so authoritarian or xenophobic) should allow you to create "livestock" slaves, that you can genetically modify to be as well "delicious". I highly recommend also "nerve stapled" to turn them into happy meals. In 2.2 you need the "slaver guild" civic to enslave your own species
So as to "how to create a species for eating":
If you are playing as a devouring swarm steps 1-4 are done automatically, but the species won't be kept as slaves but exterminated.