Can I use an existing GNSS CORS base station that I already have and stream its data via NTRIP to the Geodnet server?

No. We are using a special Crypto Chip in the miner hardware.  It is possible to make a 3rd party miner if qualified IoT or GNSS hardware company, but it is not possible to use a generic NTRIP stream from random GNSS receiver.  There is no fool proof way to fully validate an arbitrary NTRIP stream is not (1) a relay of another station or (2) faked data from another VRS CORS network.