Nix / NixOS

I'm trying to switch my home server over to nix and currently I'm struggling a bit with k3s. In a "regular" distro I would just create a registries.yaml in /etc/rancher and the rest happens on its own - that's not really how nix is supposed to work, though.

So how would I do that? There seems to be no way to create a registry using the command line (so the ConfigFlags option won't work), so pointing to the config file directly seems to be the way to go.

But how do I do that in a "nixy" way? Can I somehow say "this string here is actually a config file that's supposed to be in that dir"?