[Brainchild]

I think that there isn't much of a market for 3rd party SH Figuarts add-ons. I'd say that most of the time, the line covers all of its bases when it come to representing a character, barring some issues like Kibaranger's lack of hands or Wizard Flame Style bursting to pieces to the touch. So if 3rd parties were to make add-ons, I imagine they'd only really be there to fill the gaps that specific figures have missed.
I also think that unofficial figures would run into more legal trouble since SHF doesn't exactly allow the same artistic liberties as Transformers do.

But all of that aside, I probably wouldn't bite. I don't really care for paying premiums to enhance a figure when I could just buy another new figure with the same money.