Welcome to the Community!
Thanks for posting.
Yes, the whole home system can support up to 6 disks.
You can follow the instructions in the app, but you just need to plug the new disks into your router, with the other disks still on, wait for it to go blue, and then put it in its new location.
There's a video walking you through it here: https://youtu.be/5oekdrjheqo
Yes you can add up to 6 discs on a BT Whole Home Wi-Fi system.
But please ensure you buy the same discs as your existing BT WHW system, as the newer 'Mini' is not compatible with the original larger white discs, and vice versa.