Based on my research there seems to have been Jewish Cossacks. Based on what I have read it is possible they were Karaites who sought help of Cossacks to retrieve children and or families removed from their control by more 'traditional' Jewish communities (there is back chatter suggesting 'someone' had taken children or wives away from Karaites and they sought help to regain them).
However, the sources I have found on the internet so far don't provide much evidence. Are there any factual evidence that confirm there were Jewish Cossacks in the 17th century?