Fixed Priced Menus, as is in the name, are Menus whereby the full Menu has one overall price.
Therefore, within that Menu it generally wouldn't be possible to have the option to add extra Items on as the number of choices is controlled.
With Spoonfed, you can create, however, an Extras Section.
An Extras Section is essentially a section whereby all of the Items do not have choice constraints and are priced individually. This therefore allows you to upsell within an existing Menu for things like drinks and desserts.
To add an Extras Section, simply tick the box on the right hand side of the Section and add Items with per Item pricing.
This is perhaps better than having a separate 'A La Carte' Menu for drinks or desserts is your Customer does not have to navigate to another Menu to find these. It's also your opportunity to put extra Items on the 'shop window'.