[Answer]-Inheritance: Extending Model Classes in a sub-file

To add on additional methods to a model class, you need to set proxy=True:

from apps.main.models import Catalog

 class CatalogMasterTitleCreation(Catalog):
     class Meta:
        proxy = True
    def print_hello(self):
        print 'hello!!'

https://docs.djangoproject.com/en/dev/topics/db/models/#proxy-models