Thanks for posting the image referece.
What you want to achive requires you to copy one of the existing Spine shaders and add the dissolve effect to it on top.
You would have to create the shader via code though, as there are no Spine shader-graph nodes available.
You could also use a normal non-Spine shader, but then you will need to export atlas textures using the straight alpha workflow
and not as premultiply alpha
, as described here on the spine-unity documentation pages.