I want to edit .desktop files of some nixpkgs for customization, but they are located in /run/current-system/sw/share/applications.. and are readable only.
One possible solution would be to copy them to ~/.local/share/applications but duplicating doesnt sit right with me (this would be my last resort tho).
I would prefer to override the parameters I want to change somehow preferably in my configuration.nix only.

It understands
\nif I recall correctly. You can also write regular bash, use templating tools, etc. Just use thenativeBuildInputsparameter if you need a binary that isn’t provided by stdenv.