Is pre disk addition is a must before deploying OpenEBS?
Sure. It's supported and you need the disk attached when you setup OpenEBS as your block storage.
After you set it up, essentially you can create volumes (pvcs, pvs) for Kubernetes and mount them on your pods for consumption.
You can setup OpenEBS on Kubernetes cluster where you run your workloads either using helm or kubectl
Yes OpenEBS support storage with additional disks connected. With 0.7 it has a feature NDM (Node Disk Manager) which would monitor the disks attached to the nodes. Once the disks are attached you can create a pool on top of it and use the same. For more details, document link