You can read in this tutorial how to add a new product to the product range and then to a specific menu.

Also take a look at Product management; you can directly copy products from product overview page.

Go to Menus and select the appropriate menu to which the product should be added.
First determine the intended submenu for the new product via the drop-down icons. For example, you place a new dessert in food > desserts. Now drag the new product from the right column into the middle column so that it is added to the menu.

Before the changes take effect, you will have to transmit the changes to the location.

Add new product