You can use environment.etc to manage a file in /etc: https://search.nixos.org/options?show=environment.etc.
That seems to work - at least I can get the file to appear. Now I only have get k3s working.
Tot do that you need to start a systems service.
systemd.services.k3s = { ... }
These are the options you can set. You can inspire yourself from an existing module like deluge.
However, there already is module for k3s! All you need to do is set the options in your
configuration.nix
If there's nothing about it in nix option search your best bet is just to put the regular config file in your nix repo and have it map it in where it's meant to go