Got an answer on Discord:
You can... stop the array, click the Add Pool button, set it with 1 slot, assign the SSD to that slot. Then do the format option that will appear in the Array Operations section after you next start the array. And FYI, unRAID ships with a base section called Unassigned Devices, for any devices that haven't been assigned to the array or a pool. To make the Unassigned Devices (UD) section more useable, you need to install the UD, UD+ and UD Preclear plugins from the Apps. tab. Then enable `destructive mode` in Settings --> User Utilities --> Unassigned Devices.