All you have to do is set-up the raspbery pi with the OpenDNS upstream DNS servers:
I don't think that R7000 can push the DNS server to it's DHCP LAN clients but you could set-up the IP of the pi-hole under your Basic/Internet Settings:
The one thing to keep in mind. if you set-up a secondary DNS there (even the OpenDNS one) some queries might go out via that secondary DNS and thus miss the Pi-hole.
What I would recommend (and that's how I have it set-up) is to use the Pi-hole as your DHCP server with OpenDNS as your upstream (and disable the one on your netgear).
You will get to see each client what accesses what under the query log in your Pi-hole admin interface.
If you use the DNS in your router (see above pic) than all your queries will look like they are coming from the router (in the Query log).
You could use ddclient also for OpenDNS auto update for the IP.