You can add a VxRail cluster to a workload domain to expand the workload domain.

There are two ways (with UI and script) to add a new VxRail cluster to a workload domain, depending on your use case.
Use Case Method BOM
You have a single system vSphere Distributed Switch (vDS) used for both system and overlay traffic. SDDC Manager UI >= 4.3
You want to use an advanced NIC profile or separate vDS as overlay traffic. Workflow Optimization script >= 4.3
You have two system vSphere Distributed Switches. One is used for system traffic and one is used for overlay traffic. MultiDvsAutomator script < 4.3
You have one or two system vSphere Distributed switches for system traffic and a separate vDS for overlay traffic. MultiDvsAutomator script < 4.3