I would imagine so, albeit very few. If you converted, then you would be freed from slavery. However, you were watched to ensure that you practiced.

Vlad Tepes, aka Vlad Dracula and his brother Radu were imprisoned with their father Vald Dracul by the Ottoman Sultan, within a year Vlad Dracul was released but he supported the Hungarian King (a Catholic) during the Varna Crusades, Vlad Dracula and Radu were kept as prisoners to secure that Vlad Dracul didn't go too far in his Christian support. Eventually the boys were released Radu stayed and coverted to Islam, and Vlad Dracula went back to Wallachia and stayed an Orthodox Christian who had both Orthodox Christian and Roman Catholic allies.

[removed]

People say Frederick II might have converted to Islam, I am not one but it's muddy, but he did know Arabic and the leaders he engaged to hand over Jerusalem without a fight were impressed by his knowledge of both Islam and Arab ways that he gained from his childhood in Sicily. There was some issue when later crusaders arrived in the crusader states that they had "gone native", adopting the clothing and manners of the very hot near east rather than dying of heat stroke in traditional Frankish clothing.

It certainly happened: Robert of St. Albans being one of the more famous examples, when he joined Saladin. Leon Cazelier also famously betrayed a castle, and later converted.

The Templars even ended up with a reputation for converting to Islam, but it was mostly undeserved, since pretty much everyone had their share of traitors.

It was never a common occurrence, of course.