> Q1: Can StorageGRID as a capacity tier extension to HCI?
A1: not directly as dHCI vendors don't support SG as S3 tier
> Q2: If yes, what can be the ILM tool to move data up and down?
A2: Directly it can't be done, but indirectly you can get this done on vSphere clusters with a tool - or rather product - called ONTAP Select. For example a Nimble-based dHCI user could deploy OTS on All-Flash Nimble-backed vSphere datastores and use FP to tier cold blocks to StorageGRID.
Some of our customers who own dHCI (NetApp HCI) may have ONTAP Select included with capacity license for up to 400TB (depending on when they bought NetApp HCI storage nodes) and may be able to use it with NetApp HCI if it's still under support. The gratis ONTAP license is tied to NetApp HCI storage component and can't be moved to run off other (e.g. E-Series, or 3rd party) storage.