You'll have to ask about substitutions. As sharonabe said, some table service restaurants will let you have a small side salad, a cup of soup or possibly a veggie in place of dessert, but what is offered will be the restaurant's choice.
For an extra appetizer instead of a dessert, again, you'll have to ask, but it will be extremely unlikely at the third party restaurants because they are reimbursed by Disney according to the rules of the dining plan so they would not be reimbursed for the extra apps. An extra app, if allowed, will most likely be one of the choices mentioned above and not another app from the menu.