if you look at the main config file for core dns (named

Corefile

), you'll see two include statements: https://github.com/k3s-io/k3s/blob/master/manifests/coredns.yaml#L75-L77

the ConfigMap coredns-custom gets mounted to /etc/coredns/custom/

so you can create files names

something.override

and/or

something-else.server

in the coredns-custom ConfigMap and they will get included via those statements