Merge custom Policy with pre shipped(?)

[Xyz00777]

Hi, I'm a bit confused. When I provide a custom configuration, is the pre-shipped configuration overwritten with it?
When it's overwritten, wouldn't it be a good idea if the custom configuration from me could be merged with the preshipped, and I could overwrite the preshipped when needed instead of completely needing to copy from upstream for my own configuration and having to think about regularly updating it?
Kind regards Xyz