Spine runtimes use it's own custom animation and skin management system, so any Unity fbx exporter will have no chance to know how to get to this data.
You can however try the following workaround:
- Select the target
_SkeletonData
asset in the Project panel.
- In the Inspector, click the gear icon at the top-right and chose
Skeleton Baking
- In the window that opens, choose the desired settings and choose
Bake Skeleton
This will generate a prefab with meshes underneath it, plus animations contained below the Controller. You could give it a try using your fbx exporter.