So, you will want to take a mic output from the audio mixer into the camera.
From this page:
https://www.usa.canon.com/internet/portal/us/home/products/details/camcorders/consumer/vixia/vixia-hf-r800
It has a 3.5mm microphone input jack which can be used for getting mixed audio into the camera. It likely has the ability to turn off the microphone on the camera and just use the 3.5mm input for sound.
This will be a huge impact on the audio quality of your recordings from the camera as long as the room itself is properly mic'ed up and the mixing board operator knows what they are doing.
Testing should be performed to ensure that audio output levels are not over driving the input on the camera and that everything is working properly, but this is a fairly straightforward setup overall it looks like.
Please keep in mind, you are pairing what may be a $2,000 mixing board, plus all the microphones you are using, with a $200 video camera. It's a pretty lousy plan of attack to demand the camera to have a lot of bells and whistles associated with it. Many DSLR cameras have much, MUCH, higher quality audio inputs on it with very good lenses available that are a better way to go for quality results. But, I'm not sure what this is for, where you are using it, or what the final purpose is intended for.
