This was done on the site level, now needs to be moved to core classes:

what I thought is

to extend the whole Customer@mergeCustomer() (already extended, but referencing parent ATM)

to add a new method to the class to return a customer coping CustomerModel@getCustomer() and adding customernumber condition (cos we can’t extend the model class)

to update referencing $this->model->getCustomer() to the new method