You can expand a workload domain by adding a new VxRail cluster or adding hosts to an existing VxRail cluster.

To add a VxRail cluster to a workload domain, you can use the SDDC Manager UI or the Workflow Optimization script.
Method Details
SDDC Manager UI Supports most uses cases, including multiple vSphere distributed switches and advanced switch configuration.
Workflow Optimization script Supports all SDDC Manager UI functionality. In addition, supports custom datastore names, remote vSAN datastores, and using a JSON file for cluster configuration.

Download the script and refer to the README.md for instructions.