You can't convert culture in other subject types either, all you can do is enforce your culture on PUs, and even that just changes their primary culture. So no, it's not solvable as a simple QoL issue, it would require a whole another mechanic. People on the forum seems to be clamoring for it lately, but ultimately it's still a design choice, up to the developers, and I for one doubt that they would spend coding hours on something like this given their focus lately. Maybe for EU5, if it doesn't come with another system making this request moot.