How to change placement of CAS Accessory item.

Hi. Assigning an item to a bone does not snap it there, it just defines how it will move. So if an item is in the head area and is assigned to the head bone, it will follow the head movements. If it is re-assigned to a stigmata bone, it will move like that hand while still being in the head area.  I assume you actually want to move the mask in the hand area?
If yes, do this:
- export the blend, open;
- select the mask, switch to Edit mode ( TAB);
- select everything with A, move it in the hand area with G ( to move ) and R ( to rotate );
- click on v button and choose Delete all groups;
- create a new one with +, double-click on it to rename to b__R_Stigmata ( if in the right hand) or b__L_Stigmata ( if in the left )
- with everything still selected, click on Assign;
- expand Color attribute, delete color0;
- create a new one named the same way, set up like this

as per usual. You clone a mask, export the blend, modify it, import back, recategorize the package, if needed

Modify by following instructions in the first reply
You export .blend file from a package, modify it in Blender as explained, then import back into the package

you cannot change how the item moves in s4s, blender must be used for this