They are connected on the backend using dedicated SAS connections. Each FAS/AFF "backend" ports which connect disk shelves via SAS or NVMe as well as back end networking. There are also client/host access ports for serving out data on them.
Check out "Stage 3" on the install diagram. You'll see how the shelves are connected together.
Sales would sell you the unit with a support contract, and you would just buy the extra shelf. It might be a little longer to get a custom quote because you already have the drives. The support contract would have to be written to support those drives so you can get free RMAs for failed disks.
If you're trying to save money, it might just be better to get a pre-sold quote and return the drives from wherever you bought tuem (unless they are lightly used drives). Chances are the drives may have some hours on them and be closer to hitting the end of their useful life, and the account team may not let that happen. If they came from another system you had that you bought from NetApp directly (or through a VAR), that's a different story.