I'm afraid this will not easily be possible, as the Spine shaders are not surface shaders but full shaders with vertex and fragment shader stages. So depending on what you want to achieve, when you are already extending the shader, it might be easier to create a new graph with the lighting/etc nodes that you want. Spine's input only differs by using premultiplied-alpha by default, so you might want to set blend mode to One OneMinusSrcAlpha as in the shader, or switch your Spine workflow to straight alpha.
We might add some shadergraph versions of Spine shaders in the future, but I'm afraid this will take some time until we get to implement this feature.
