Good to see you here! I don’t use Softimage and haven’t touch importing a skeletal mesh yet. It might be helpful if you post your question in the AnswerHub( https://answers.unrealengine.com/ ) along with a picture of your rig and its hierarchy or better yet a scene file or FBX so someone at Epic can look at it.
Not use if this is useful?: Softimage to Unreal Engine 4 using Exocortex Species - YouTube