Yes, this is what i did. "ext client dns" is the netmaker server x.x.x.254. But coredns dont resolve names from external adresses. "custom dns entries" works properly with coredns. Corefile:

. {
    reload 15s
    hosts /root/dnsconfig/netmaker.hosts {
    fallthrough    
    }
    forward . 1.1.1.1 8.8.8.8 
    log
}