Finding Unknown DNS Servers

I am looking for unknown dns servers. I'm attempting to use NetFlow to identify DNS traffic on Router1 and wanted to use endpoint filter for the ones I know about and noticed that there is an IP Address Group filter but it only offers RFC1918 groups. Is it possible to define a KITTIES group with and a PUPPIES group that is that would show up in the IP Address Group section for selection?

Or if you know of another way to find unknown DNS servers it would be appreciated.