I have an existing CoreDNS cluster built outside of k8s cluster, can k8s just use the existing one, without the CoreDNS deployed by kubeadm?
Yes you can do this 
Just remember, CoreDNS is used to solve internal Kubernetes Services to ClusterIP (or Endpoints, if using headless service) IPs, so anyone with access to this CoreDNS will resolve this name.
Also, make sure you configure the option —cluster-dns in your Kubelet cmd startup to point to the right DNS address, and that this DNS is reachable by all Pods