On the order page add a listview with all products from the database directly.
In the listview add a button that will trigger a MF to create an orderline, associate the orderline with the order and the product from the list view. Also add a listview with order lines on the order so that the new orderlines will be shown on the order.
Like this for Example?
As you can see i changed the products page into a listview with a button
But I have trouble Envisioning how i would Enter this information to an order, and how to microflow would look like.