I've never heard of any saint having an official patronage for the conversion of sinners; it's an interesting question.
In our present time, I suppose we should note that the conversion of sinners is a prominent theme in the apparitions of Fatima both the appearances of our Lady, and earlier the apparitions of angels which Lucia, Jacinta, and Francesco witnessed.
So it would certainly be appropriate to observe devotion to our Lady of Fatima for the intention of the conversion of sinners. [From the Vatican]
St. Hubert was known for converting lots of people, even if it's not an official patronage title: