The question is not about a library or tool, but about the specifications (either the standard or the summarized definition) of a YUV 4:2:0 movie stream.

Are YUV 420 movie streams just some concatenated YUV images, and if so, what is the specification of these frames and of the stream?

I want to make a simple images to YUV 420 helper, but if it happens that it requires also some computations / compression / prediction, I'll just surrender. If it just requires to convert images to YUV then append them together, I'll code it and share the C# source here.