If it's in his nature, the Penance may be able to help him by giving him damned souls from Hell itself to eat. If that wouldn't help or satisfy him, then I think it could be a grey area in this case, punishment-wise

For instance, if he murders someone and it's against their will, more time will be added just as it will be for anyone. If it's mutually agreed upon and the victim sees it as a form of voluntary punishment/absolution for their own sins, then there probably won't be any additional time added on at all.

Hope that helps!