All array drives in Unraid need to be formatted by Unraid so you cannot simply add the drive to the array. You could, however, easily mount it outside the array using the Unassigned Devices (UD) plugin.
What is not clear to me is where you want to end up? For instance do you intend to continue to run Windows in a VM under Unraid? If so you may want to keep the SSD as a Windows boot drive mounted using UD. In such a case since having at least one array drive is mandatory in Unraid you could assign a second USB stick to this role if it is not going to be used to store data.