How can I create a Business Data catalog field with the object model? So I want to create a field that utilities business data catalog and not add an item to an already existing list.
You will have to be a bit more precise. BDC fields are used in many places and can be defined in many ways: application definitions, user profiles, lists, lobsystems in web parts etc.
below is an example on how to instantiate a BDC field in an SPList, as this is something that usually causes a bit of trouble: