Dummy boards, dummying down, etc. All a means of giving the colony the space it requires, while doing something about the void that may be left. Generally we fill it with insulation of some kind.
With your nuc I would put it into the hive and place it against one side. Give them a frame or two of foundation on the 'open side', the dummy board and fill the space with celotex/kingspan, making sure to tape cut edges. Give them a couple of pints of invert/syrup, remove insulation and add more frames as they expand.
You may consider leaving them in the nuc and adding a box on top with another six frames. They do well over Winter in this set up.