IK is what I call "hierarchy + bones + weighting + influence" for laziness reasons ,) IK itself is obviously just a solver...

Optimally we'd need a 3dsmax gr2 importer/exporter plugin

TES modding only took off when the Nif tools allowed you to import an original source mesh + skeleton and bone/weighting, see how it worked, apply changes, and see how changes applied to the game.
The GR2 viewer makes it possible to see the skeleton, but we can't export a mesh bound to that skeleton...
In the end, I feel like it might be easier to just ask Larian directly... maybe their animator and model artists tell us how to do it...