Hi - thanks for getting in touch - could you please take a look here and see if this answers your questions?
https://learn.microsoft.com/en-us/windows-server/storage/storage-spaces/add-nodes#adding-drives
This should walk you through the process, although once you've added the drives, you should be able to add them to an existing pool, and S2D will take care of the rest, at which point you can extend your existing volumes etc.
Thanks!