NixOS needs to handle generating the config to make service configurations composable. If you don't want that, you might as well write your own systemd unit.

That being said, most services have an option to use verbatim configuration files[0] or fragments[1][2][3].

[0]: https://search.nixos.org/options?show=services.yggdrasil.con... [1]: https://search.nixos.org/options?show=services.nginx.config [2]: https://search.nixos.org/options?show=services.bind.extraCon... [3]: https://search.nixos.org/options?show=services.nginx.appendC...