Whether FSSP or otherwise, it would depend. For one thing, what field is the grad degree in? For another, there are certain prerequisites that would have to be satisfied irrespective of the degree itself. It would also, of course, depend on whether the candidate is accepted, and again, that irrespective of a graduate degree. All of that said, IMO an anonymous internet forum really isn’t the place for such a question. Best, I think, to present the details to the FSSP vocations director and see what he has to say.