Yes, copy-paste an existing personality from Stellaris\common\personalities and make sure it has a higher weight than the default personalities if your conditions are met.
If you want to attach the personality to a specific, prescripted empire, add a country flag to the empire in question (like the human prescripted empires do in Stellaris\prescripted_countries\00_top_countries.txt) and use that as your condition.
If you do that, you probably also want to check for fitting ethics still being present, since empires can change a lot over the course of a game. That empire that started as a fanatic militarist may very well become a pacifist over the course of a century, and its custom personality may not fit anymore. Much better to allow standard personalities to take over at that point.