You can use Extensibility in Smart Cloud as it works in exactly the same way as SDx, however, some infrastructure work is required by the Smart Cloud Support Team. The Smart Cloud Support Team must set up an initial estate to get the Cloud server talking to your company's infrastructure on-premise system. Once the configuration is done, Extensibility for both the server side and the client side is exactly the same.
For information on contacting Smart Cloud Support Team, see Contact the Smart Cloud Support Team.