Products can be added to the working order from Linesheets just as they would be from the Product Gallery.
After clicking on a specific product to view the product details, the product can then be added to your Working Order from there.
Just click on the "Add to Order" button from the product details panel.
Additionally, you can use the cart icon to add the product directly from the linesheet.
You can also add products to your order in bulk by selecting the check box in the image's top left corner. Once you've selected the items you'd like to add select the three dots next to "Actions" and click "Add to Order" from the drop down menu.