It is possible to declare a subclass of an existing MT::Object class,
one that shares the same table storage as the parent class. Examples of
MT::Category. In these cases,
the subclass identifies a ‘class_type’ property. The parent class must also
have a column where this identifier is stored. Upon loading records from the
table, the object is reblessed into the appropriate package.
Class->addclass( $typeid, $class )
This method can be called directly to register a new subclass type and package for the base class.
MT::Foo->add_class( 'foochild' => 'MT::Foo::Subclass' );