How do you create a custom stockpile that accepts only prepared foods?
On the stockile menu, select for Custom Settings. Disable all categories except "Food" and "Additional Options" (press on each other category), then highlight "Food" and press to disable everything classified as food. Finally, press to enable the "Prepared Food" option on the lower right. Now hit and draw your custom prepared food stockpile as normal.
Note that some stacks of prepared food are too large to fit in a barrel (or pot), so it is advisable to limit the total number of barrels in the stockpile (from the uery menu, press to reduce Max Barrel) to ensure that you always have a few barrel-free stockpile tiles for oversize meal stacks. (Dwarves are not smart enough to move an empty barrel to store a stack of 300+ masterful roasts, let alone consolidate partially-full barrels to make room.)