Hi Eva!
I usually refer to this kind of organization:
Slot: hand
Skin: hero / evil
Skin placeholder: hand-open-front / hand-open-side / hand-closed-front
Attachments: hand-open-front-hero / hand-open-side-hero / hand-closed-front-hero / hand-open-front-evil / hand-open-side-evil / hand-closed-front-evil
Therefore I place the poses in the same slots as different skin placeholders. And I activate the correct pose during the animation as usual.
I hope this helps!