Looks a lovely car.
It will be an expensive retrofit. You'll need the seatbelts, seatbelt loom, seats, mounting points (metal bracing). Plus you'll have to change the v5 document.
I'd recommend you speak to Audi retrofits. Or alternatively, find a q7 in a breakers yard and buy the whole kit.
My old A6 is a 7 seater, and I know the conversion is quite involved and not cheap to do.
This link should help:
https://www.audiworld.com/forums/q7-discussion-112/q7-5-seater-7-seater-2939656/