By what method? I don't think you mean convert them by force, which is not conversion. What Islam needs is another 'holy prophet' who will re-write the koran and take all the crap out of it. Short of that, I suggest they isolate themselves and let them 'purify' themselves without sacrificing any more non-islamic blood to quench 'allah's' thirst.
When they are swimming knee-deep in their own blood and stumbling over their corpses they might begin to look for a more benevolent and humane 'god.'
As it is, even the beasts of the field have a better form of worship, which is to say that no 'god' at all is better than the bloodthirsty tyrant they have now.
Christian missionaries have gone into numerous countries and converted people to Christianity, including people who belonged to world-class religions such as Hinduism and Buddhism. Many have been martyred in the process, but the spread of Christianity has been extremely successful, because it is fundamentally an attractive, hopeful, and beneficial religion.
The one great exception is Islam. There have been virtually no conversions from Islam in any country where Muslims are in power, because conversion is simply not permitted.
Even in Communist China, Christianity is currently spreading and gaining new converts, despite the prison camps and the thorough brutality of a Christian-hating regime. But not in any of the Muslim countries. Somehow, that has got to change.