I need to know how to add a page to a saved document.
How do I add a page to a saved document?
Not at all - by whatever means (macro, GUI,…) you need to open the document in order to make changes and you need to save the document afterwards.
Carry on writing for LibreOffice to create pages a needed, or click at the end of the document and then press Ctrl+ Enter to create a page break. Click Insert - Break - More breaks for other breaks.