hai, the period when most conversions christian -> muslim took place in Hispania was well before 1066, specially just after the islamic conquest (8th century); of course there could be conversions later, but not "en masse"; in the period covered in the game, as far as Hispania is concerned, there were usually conversions muslim -> christian. But, turning to the original question, as you can only play christian kingdoms, the possible conversions you can force would be muslim -> christian and pagan -> christian (maybe also jews -> christian?).
Marc