It's been said: even JWs do not teach that baptism washes away sins. More like baptism being a request to God for a clean conscience (of course, there are spins on that in the JW fashion). That the candidate got baptized is an indication that that request was granted. But we know this is not necessarily so, even according to JW tradition. A baptism may be nullified if the conditions for nullification are fulfilled. A person requesting a clean conscience may be involving in acts betraying a soul in search of a clean conscience. In the JW world, a disfeloowshipped person is in no position to request for a clean conscience until he is reinstated by a ciommittee of men. He is dead. Hiow can the dead seek forgiveness? Only those who have previously not fallen away from the way of the truth can correctly do that, and DF persons are not of that class. Ideally. But I have heard of someone who did just that: moved; attended meetings; studied again; got baptised; and 'progressed'; I don't know how it all turned out though. It is difficult to understand how the elders, chosen by and under the influence of the holy spirit, could have been deceived in this way...
So, it could be done, but will still be interpreted as illegitimate by the JW community. He would have a status of being in 'good standing' until what had happened gets known. From then on, anything may happen. He may be forgiven if extunuating circumstances are cited, he may get punished for making the lie of God's princes, the elders...