This was found years ago and somehow got not fixed, but there's an
easy c/p link here.
But to make it easier, I just made a
world_default.xml file version with the Punjab province added to both India and Pakistan (per that thread) and I didn't update the populations or whatever, but figure you can do that in-game if you really cared. You'll need to add the .xml extension for it to work for your game but otherwise should be good.
I was able to replace the data in my existing file (which is heavily modded already) and it imported without any issues. You will need to replace existing data though, don't just "update/add" or else it won't likely work. Will need to restart for your flags to reappear.