I'd be a little suspicious of this "converted priest" story, too. However, Islam has always had a peculiar attraction for a certain type of man - one of them being its strange attitude toward sex. For one thing, it regards pederasty as perfectly okay, and sees teenage boys as the chosen sex object of adult men. This was one of its attractions for the 19th and early 20th century Etonian types who converted or at least went to live in Muslim countries.
It is basically a fine religion if you're a man and the most important thing in your life is what's between your legs. Remember that one of the late unlamented Ayatollah Khomenei's first published works was a treatise defending the legitimacy in Islam of men having - er, a roll in the hay - with chickens.
Women come after teenage boys and chickens, and I seriously think that is one of its attractions for some of our loopier and more amoral Westerners. I wonder if it might not be one of the attractions of Islam in prisons, too - it makes these men feel that they're all powerful, they have a right to lord it over people, simply by being Muslim and by being male, and by being prone to violence (which is the other major component of Islam).
I think some of the attraction to Islam is the indifferentism that has infiltrated Christianity in the West. Many Muslims still have a complete devotion to faith, as opposed to the Christians they see watering down their faith. I think the Muslim conversions in prisons are often because of the vacuum left by poor evangelism by Christians in prison.