Unfortunately this is not possible. Megaventory calculates the prices on every line, you cannot add a discount that affects all lines.
It is however possible, to define a product named "discount" and then change its price manually to a negative value when you add it in a document. That way you can apply a total discount for the entire document and all its lines in one go.
Note that for this you will need to give access to edit prices to the user who can add this negative value. This will also allow him to change the price of any product in the document (essentially they can override the default value).