To expand a storage pool without using Hitachi Dynamic Provisioning, add SDs to the storage pool. If the storage pool is tiered, you can add SDs to one tier without adding SDs to the other tier. To see how full each tier is, use the span-list --sds command.
If you are using Hitachi Dynamic Provisioning, see the Hitachi NAS Platform HDP Best Practices (MK-92HNAS063) for recommendations.