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