String src = dataDir + "Page numbers.docx";
String dst = dataDir + "Page numbers_out.docx";

// Create a new document or load from disk
Aspose.Words.Document doc = new Aspose.Words.Document(src);
// Create a document builder
Aspose.Words.DocumentBuilder builder = new DocumentBuilder(doc);
// Go to the primary footer
builder.MoveToHeaderFooter(HeaderFooterType.FooterPrimary);
// Add fields for current page number
builder.InsertField("PAGE", "");
// Add any custom text
builder.Write(" / ");
// Add field for total page numbers in document
builder.InsertField("NUMPAGES", "");

// Import new document
Aspose.Words.Document newDoc = new Aspose.Words.Document(dataDir + "new.docx");
// Link the header/footer of first section to previous document
newDoc.FirstSection.HeadersFooters.LinkToPrevious(true);
doc.AppendDocument(newDoc, ImportFormatMode.UseDestinationStyles);
// Save the document
doc.Save(dst);