Hi,
Since Spine renders using Unity's mesh-based Renderer components, there is no kind of "current frame" image available for a given Skeleton out of the box.
What you want to achieve is render a certain state of a mesh down to a texture, which can be achieved in Unity by using a RenderTexture. You can find a short description at the bottom of this thread (regarding transparency fade-out of a character):
Help plz! How do I make my character translucent in Unity?
Maybe there is a package available on the Unity Asset Store to do this more comfortably, search tags could be "render texture" or "impostor".
Maybe this package would be interesting, it is not free and I haven't tested it though:
https://assetstore.unity.com/packages/tools/utilities/amplify-impostors-beta-119877