You want to add SSDs to existing HDD aggregates.

 To add SSDs to existing HDD aggregates, you need to use the Automated Workload Analyzer (AWA) tool, which is a part of the Active IQ Performance Analytics service. AWA can help you to validate how many SSDs would improve performance by analyzing the workload characteristics and performance metrics of your cluster and providing recommendations for optimal SSD allocation. AWA can also help you to identify the best candidates for Flash Pool conversion, which is a feature that allows you to combine SSDs and HDDs in the same aggregate to improve performance and efficiency12.

OnCommand Workflow Automation is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for automating storage management tasks and workflows3.

Active IQ Unified Manager is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for monitoring and managing the health, performance, and configuration of the cluster, not the SSD allocation4.

ONTAP System Manager is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for performing basic administration and management tasks on the cluster, not the SSD optimization5. References :- Active IQ Performance Analytics, Flash Pool overview, OnCommand Workflow Automation overview, Active IQ Unified Manager overview, ONTAP System Manager overview.