First off, Marvel owns all of the characters. What Fox has are the film rights and related rights to the X-Men.
What specific rights they have are defined by the terms of the particular contract that was signed by all parties. But typically, what that amounts to is either a specific list of characters, or some blanket provision that they get those media rights to characters who originated in X-MEN or related books.
So no, you couldn’t declare Wolverine or Iceman or Storm an Inhuman and suddenly have Fox no longer able to use them, nor would that give Marvel or any other studio the rights to use Inhuman Wolverine, etc.
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