You do that using the Order AnyWhere feature. Go to Merchandising > Order AnyWhere, select the product you want to have the add to cart button for and View the selected html. Cut that and paste it on the page where you what the add to cart to be.
Order Anywhere links are designed for people who want to add ecommerce to their hand created pages. Since the pages are not created by ShopSite there is no relationship between what is on them and the data in the ShopSite database. So if you change price on something in ShopSite you will need to manually edit your other pages to update those price changes.
If you want the prices to automatically reflect what you set in ShopSite you will need to use ShopSite to generate those pages.