hi siso I think this question is incorrect . because this is the sentence of active voice .I think the question was to change into passive voice.

The answer is

Guilty had been found by Joseph .



The reason the question is active voice is because the (the subject Joseph) is in the start . !{Joseph had been found guilty}

so the question is in active voice and you have to change it into passive voice (Guilty had been found by Joseph)​