>… how could I add this?
I dont know your exact scenario
but i would have thought you can just set it up like any other menu item and just label it like $x.xx per portion instead
of the usual “small” (or whatever else you want to say in the sizes settings)

I dont offer custom development (https://docs.wp-pizza.com/faqs/?section=customisation) , but
there is plenty of documentation regarding filter, actions etc here
https://docs.wp-pizza.com/developers/
that can be used for all sorts of modifications i would have thought

easiest seems to me to name it “multiples of 3” or some such, then put in the description of the item something that explains how it works i.e
upto 3 -> 10.00
upto 6 -> 20.00
upto 9 -> 30.00
etc
so if you add just one of those “products” the price is 10, add another one, price is 20 etc etc

alternatively you can do something based on those ideas here
https://demo.wp-pizza.com/wppizza-add-ingredients/our-menu/user/
for instance the “chicken curry” could be “upto3”, “4-6” and “7-9” (or whatever your steps are) with the prices set as needed

or go even further and do something along the lines of the “Platters” on that same page

all seems quite doable to me…