is it for anonymous users or logged in users?
when a req flows through DAF pipeline it creates 2 session object - Profile and Cart.
cart.getCurrent will give u the current order.
if the user is logged in u can call cart.getSaved to see if an order is already there for the user. U can set the saved order to current order.
u can merge the commerce items calling mergeOrdersCopyCommerceItem method of CommerceItemManager.
u can get reference for CommerceItemManager from OrderManager.