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.