Let me then ask a question: does God call Eastern Orthodox men to the priesthood? The EOs are not in union with Rome, but whenever an individual EO priest does swim the Tiber, he swims in his vestments, i.e., he is received as a priest.
That said, would God call a Catholic man to the EO priesthood? Obviously not, for God would not call a man into schism. Would God call a Catholic man to the SSPX? Obviously not, due to their irregularities.
Let’s consider, though, the state of a man who was raised by a family frequenting an SSPX chapel. Let’s say he admires the priests of the SSPX and decides to go to its seminary and become ordained. His ordination is valid, no doubt, yet illicit according to canon law. During all of this time, he has no qualms about not being in union with Rome, but ardently hopes that such will be the case. Until one day, whether through his learning or his reflection or an interior prompting, he comes to the conclusion, to which he fully assents, that to exercise his priesthood outside of that union is sinful. From then on, full knowledge and deliberate intent being there, there is a case that mortal sin is being committed, for he knows now that he is saying Mass illicitly, and thus in a state of disobedience to Mother Church. He is not, however, committing sacrilege, for the Church would never deny his priesthood.
His obligation would then be to contact the bishop of the place where he is serving, express a desire to incardinate in his diocese, and ask his counsels. He is then obliged to obey the bishop. If the bishop advises him to continue to administer the sacraments at the SSPX chapel despite their illicit state until he receives word back from the Holy See, he is obliged to do so. If the bishop orders him to cease immediately and to report to the diocesan offices to consult with him and his vicar-general, he must do so.
I hope that answers the question.