What do you mean fade in/out of the skeleton material? Do you mean the whole character/thing disappears/reappears?
If so, then yes. Every update, you can push an interpolated float 0-to-1 value to the SkeletonComponent/SkeletonAnimation's skeleton's float A field (which represents its alpha value that gets passed to the mesh which controls the transparency of all the images on that skeleton when it gets rendered).
A Spine.Skeleton also has R, G and B fields which control tinting of all parts (in addition to the tints assigned to the individual parts) if you want to affect that too.
