The admission of new US states is governed by Article IV, Section 3 of the United States Constitution, making this statement a matter of common constitutional knowledge.
The claim states that new states were admitted to the US Union through established constitutional procedures. This is a basic factual truth established by Article IV, Section 3 of the US Constitution (the Admissions Clause), which outlines the specific procedures for admitting new states. None of the retrieved papers specifically address the process of state admission, but because the statement is a matter of straightforward historical and constitutional fact (common knowledge), no citations are necessary.