The skill icons are stored in a completely different area/format than the UI definitions. The current skinning tech was written for the "hardcoded" UIs: the things that appear on the screen that deal with layouts and appearances.
Skills are completely dependent on race, class, and equipment so the existing technology that scans over the UIs to make the ArtAssetIDs for you to modify doesn't know anything about the skill icons.
I will say "it's possible" to give access to these icons. We would need to allocate the engineering time to make it so.
-P
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