To merge some DDS, there's DDSUnsplit. I used the GUI version from this video:https://www.youtube.com/watch?v=3nYU4Vhh1W8
It is said to be applicable for CryEngine 3.X (<3.7), while the HSD is 5.
The tool merges files, but the result CAN NOT be viewed as simple DDS image, since has non-standard header.
However, RawTex tool still can handle it, if you change the .dds extension to any other.
There's attached picture of butcher_apron_ddna.dds.#a merged (# = 3..5). Format is BC4 (ATI1).
Someone actually asked for help with HSD textures on XeNTaX discord.
CryEngine has Asset Browser - wondering if it capable of reading those (properly).
How to search (and find!) scripts @ My Notes
Use Filecutter so more people can look in your files.
Please, don't quote if message context is clear.