When I ordered my server with Hetzner, I only loaded two NVMe SSDs into it. As I grew into my server and needed more storage, I contacted their support to add additional drives. Details and prices for the drives can be viewed at this link, and whatever the server configurator allows for an upgrade is possible to add at a later point.
When the first additional drive was added into my server, I was able to set it up from within Proxmox:
Click on the Proxmox node → Disks → new drive that’s been added → Initialize Disk with GPT
Afterwards, click on Disks → LVM → Create Volume Group
You should be able to select your freshly initialized disk, name it what you want, and then enable Add Storage and click on Create.
Afterwards, click on Disks → Directory → Create: Directory.
You should be able to select your freshly initialized disk, select ext4 (or xfs if you prefer), name it what you want, enable Add Storage, and then click on Create. The name that you choose will then be a part of the path to this directory: