Instead of adding an extent to a VMFS datastore, you can increase the size of an existing datastore. Use the vmkfstools -G command.

You might increase the datastore size after the underlying storage had its capacity increased.

The command uses the following option:

-G|--growfs device device
This option expands the VMFS datastore or its specific extent. For example,
vmkfstools --growfs /vmfs/devices/disks/disk_ID:1 /vmfs/devices/disks/disk_ID:1