Short answer is no, not yet but we are hoping to in the future. So if you want to feed your bees over winter with fondant, there are a number of suggestions we advise.
1. Use the frame feeder to put the fondant in. Simple and it works but ideally fondant is better when directly on top of the frames where it is closer to the bees when it gets really cold, not to one side. It should work though if the colony is a good size (and it is in a poly hive so it will be warmer for them).
2. Use the langstroth nuc extension box instead: https://paynesbeefarm.co.uk/collections/poly-langstroth-nuc/products/extension-box-for-langs-poly-nuc
Ok, so it will be deeper than it needs to be but this works. You can put the fondant straight on top of the frames and the bees will be nice and cosy in side still.
3. Or you can get away with using a poly nuc eke for the national nuc: https://paynesbeefarm.co.uk/collections/poly-nuc-national/products/poly-eke-for-poly-nuc-hive
Although it won't fit on properly, it will just about fit on enough to cover any entry points and keep them wasp proof. The langstroth nuc lid will fit nicely on the top. However, because the eke does only just cover, it means some of the points are thin adn you may find the bees will nibble their own entrance at some of the joints.
Dispite this, we consider this the best solution as it allows you to put fondant directly above the frames but without giving them a huge amount of wasted space.