What I see in practice is when I open a Pages '09 or Word document in any version of Pages since Oct 2013, the translated Pages document bears a single segment header/footer as that is the style of the donor documents.
Since Pages only supports one template style per document, you cannot have a document with mixed single and triple-segment headers and footers. If your current and donor documents are from any version of Pages since that Oct 2013 date, then you will have mandatory triple-segment headers and footers and no means to convert them to single segments.
As a test, I just opened a Word .docx document in Pages v14.3 on Sequoia v15.3.1. I opened another Pages document containing multiple sections and copied/pasted that content below the existing translated Word content in the first Pages document. All of the pasted material resulted in a single-segment header/footer owing to the inheritance of the existing document style.