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 10.110.192.0/18 and a PUPPIES group that is 10.80.0.0/16 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.