You take the animation set file for skeletons:
\_(AnimSet)\Creatures\Necropolis\SkeletonArcher-arena.(AnimSet).xdb
\_(AnimSet)\Creatures\Necropolis\SkeletonWarrior-arena.(AnimSet).xdb
\Characters\Creatures\Necropolis\AlternativeUpgrade\SkeletonWarrior-arena.xdb
Each file contains a list of animations. You need to add an entry with the parameters Kind = specability and copy the Anim href from the most suitable entry from another part of the same file.

With all these changes, the animation for the skeletal warriors might break, because it also references the specability or attack00/attack01 animation if it's missing.