Anything’s possible! 😉
But this is not something WordPress will let you do out of the box, however. You’ll need a plugin for this, an existing one or one you write yourself.
I guess you’d want to have a button or maybe a special choice (“New…”) in your Toppings drop down menu. When selected, it could open a pop-up and load the edit screen for a custom Topping post type. You likely would want a simplified edit screen here, so it doesn’t get too confusing.
You would save back the new Topping using ajax and, upon a successful save, you would add the new Topping to your current Pizza’s list of toppings.
Does that help?