It is not possible to add extra pages to the card. However, if you have chosen a flat (2 page) design, you can often get the same design in folded format (4 pages) by going back to the page with the designs and selecting a different format.
If you are looking for blank pages inside the card, please note that the card will be printed as you see it in the design editor and the preview in the shopping basket. If you see a blank page, the page will remain blank when printed. if there is a pattern or background, this will be printed and cannot be removed.