I never even considered let alone try to steal health for anyone else minion so I don't know. I would hope you can't. I know I would not appreciate someone coming along and off a minion I cast to help me just to steal some health points. IMHO if they need health points they should stock enough heal cards to take care of themselves.
I know you are just curious and looking for answers and I am not implying you would do such a thing. Just giving my own thoughts on the topic. I hope someone else can answer on whether they can or not.