The dog pen, and cardboard box would be a good idea. Put the cardboard box in the dog pen if it will fit, or attach it to the dog pen. This would be a VERY GOOD IDEA. You could get the birds out of the house immediately and set them up insight of the birds but out of reach. Leave them in this set up for two weeks. Taking the box into the coop at night. It needs to be of good size, like 2 x 3 feet box. Do not feed them in the box, but rather outside in the dog pen.
Some people have had good luck, putting the little chicks feedbowl on inside the pen, and the adult feed along side of it outside the fence, then they get used to eating together.
A lot of people do integration this way, this may be your best bet. Inside the run, the dog crate (size?) with a heavy duty box either attached to the gate of the crate, or inside the crate for day time protection from the wind. Do make sure it is secure. Using thick rubber band (hardware store) with hooks on each end, could easily attach it. At night after the old biddies have gone to bed, move the box into the coop for the night, outside each day.
Mrs K