Is it permissible for a Muslim to claim that he was not a Muslim and recently converted to Islam in order to encourage Muslims to adhere to their religion and encourage polytheists to embrace Islam?

It is not permissible for a Muslim to claim or feign polytheism (shirk) except out of necessity or coercion, because that is a lie and deception and may lead to disbelief (kufr) if he utters a word of disbelief or claims disbelief. If he is ignorant of the ruling on this, he does not become a disbeliever. A good intention does not justify a prohibited means, therefore, one must repent to Allah and not return to such an act.