The provision concerning states admitted to the United States out of the territories is that “such state shall be admitted, by its delegates, into the Congress of the United States, on an equal footing with the original states in all respects whatever…” The text is in Art. 5 of the Northwest Ordinance:
https://www.ourdocuments.gov/doc.php?flash=true&doc=8&page=transcript
Whether or not new states could secede raises only the same issues as those for the original states.
