Since no one else has responded yet, I thought I'd give you a bump and tell you what I have read on the forum about similar situations.
You might be successful if you place the 1 week old under the hen at night. Then, make sure you are observing them first thing in the morning, as soon as they wake up for possible pecking or in case the hen doesn't accept the chick and decides it is a threat to her new babies. Be prepared to grab the chick quickly in case things go wrong. If the first several minutes pass, without any problems, continue watching to see that the older chick doesn't pick on the new babies.
I've never done this and I hope someone with more experience chimes in soon...but I think that is what I would try, based on what I have read of other members in this type of a situation.
