move items to wppizza menu items

this is most likely not going to work with all the data (as the meta data and other things will be different), but as a starting point, you could export your already entered items (tools->export) edit the resulting file as required and then import

(you probably want to also export the wppizza items to have a look as to how the data is organized there )

before you do any of that though, I strongly recommend you make a backup and test first