Hello, and welcome to the Flow forum! 
I don’t know what a “uniting board” is, but you can adapt National frames to fit inside Langstroth frames. It is a little untidy, but the bees don’t care. Once the colony is established, you can slowly swap out the modified frames for 100% Langstroth. Here is the post describing the very quick fix to do this:
Hope that helps. If not, you can use the magnifying glass search tool at the top of the page to search the forum for more ingenious ideas, or just ask another question! 