If you CHANGE the food at all, you have to repackage it.
EDIT: There IS a term for this, it's called "reverse-palming-off." There are limitations on what you can do, in the absence of a contractual agreement with the original manufacturer.
My original comment related to the idea of buying a "Ceazar's pizza," adding something (say...salt, feta, basil), re-baking it. Then it is no longer a Ceazar's pizza and you can't remarket it as such.
There are certainly also many examples of people who re-sell packaged food: Businesses that put together "gift baskets" which contain a variety of specialty items, such as cheeses, mustard, crackers, wine, etc.
The legal issue is two-fold: "are you representing some product made by another purveyor as your own?" And the converse: "are you adulterating/changing some other purveyor's product and still representing it as being their product?"
One of the early cases on this discussed taking a cask of whiskey and breaking it down into smaller bottles. Can you still market it as "Jim Beam(TM)" whiskey? The court decided you could. If however, you watered it down (adulterated it) then you could not.