You used to be able to encourage this by giving them a governor of the appropriate kind. Now that they pick their own governor, I have no idea.
I feel like for CNs especially, we should be able to issue 'directives' that encourage the AI to behave in a certain way (like how AI personalities work). It's good that you don't have to micromanage CNs, but it would make a big difference if we could optionally tell them to be more or less expansionist, or to specialise more or less in production/trade, rather than leaving them completely at the whims of the AI personality lottery.