You won't see LUNs from an expansion unit in the modular server GUI. It will only pass through commands.
How do I configure my external RAID subsystem through the GUI? Do I have to attach it to both SCMs if I run in dual-mode?
The external storage array management software must be used to configure all drives within the array. Please see the tested hardware and operating system list (THOL) for the list of peripherals that can be attached to the external SAS port. You must order external RAID subsystem units with a second controller to be able to attach to both SCMs simultaneously.
and
How do I connect external devices to the SCM?
View the back of the chassis, and select the SCM. Select Actions - Expansion Port. Choose the module that will use the external device, and click Apply.
See the Promise* VTrak Connection Guide for more information.
from Intel® Modular Server System MFSYS25 — FAQs about the Intel® Storage Controller AXXSCM3S