Hello,
Why not simply check the manual.
K.15.16.x is one of the current firmware releases. I have chosed the management and configuration guide for this release (there are different manuals for different releases, because they contain different features)
http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c04490719-2.pdf
Go to page 157 to 169.
On page 166 you can find a list of restrictions, but I dont see a restriction there that only servers are supported as downstream devices.
Checking older release notes I see the following enhancement introduced in K.15.05.0001 (this release should be about 4 years old)
Distributed Trunking Switch-to-Switch
■ Enhancement (PR_0000063613) - Adds support for switch-to-switch Distributed Trunking. See “Port Trunking” in
the Management and Configuration Guide.
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=c03315372
HTH