I have just got a short paper (about 3,500 words) accepted by a top journal and it is currently being copy-edited. Meanwhile I have completed a paper (about 12,000 words) with deeper research that contains the short paper as a whole. The new paper reorganizes and expands on the short paper in a way that improves on the accuracy, although it takes much of the short paper contents verbatim. Then it adds an entirely new section. Is it inappropriate to submit this to a journal?
It is quite common, to publish a short letter-like paper to a high-impact journal first and then follow it up with a more detailed longer paper expanding on the first one.
However, containing the first paper as a subset is (self-)plagiarism, not original, and not acceptable. You must write the second paper independently, though some similarity of the text is okay, in particular if you state explicitly which passage has already been presented in that earlier paper and are only included for completeness.