I think you may be referring to the "janissary" who were employed by the Turks as corsairs operating primarily along the barbary coast. Many of these janissary were recruited from Christian portions of the Turkish empire like Greece and the Balkans. Barbarosa was the most famous of these.
The janissaries were, at least originally, Christian slaves levied from the Turkish lands in the Balkans. They were converted to Islam and composed for a couple of centuries the most efficient army units in Europe or the Middle East.
They gradually became corrupt and even hereditary, eventually winding up as a sort of armed mob that deposed deys, beys and sultans as they chose.
But what I was talking about were actual adult Christian men who chose to join the corsairs, either after being captured or out of ambition. For the last couple of centuries of the corsairs' existence these men dominated the corsairs, as the natives slipped farther and farther behind technologically.
Barbarossa was the son of a janissary, but he was raised a Muslim rather than being "drafted" as a child from the Christian natives.