Hello,
UPDATE: addHeaderSection and addFooterSection allow adding headers and footers per section. These methods include the removeOthers option to keep or remove existing headers and footers when adding new ones.
importHeadersAndFooters remove existing headers/footers to add the imported ones. There's no method to add new headers and footers keeping existing ones in a DOCX document. A custom development should be done for this specific task.
Using template methods you can replace placeholders in headers and footers (and also other targets: document, footnotes, comments...). Using DOCXPath you can add, remove and replace contents in the main document, headers and footers; but headers and footers must exist to be able to change them.
Regards.