I'm sorry, I don't exactly understand what it is you are asking. In your example code, you are NOT sub-typing. Do you want to create a sub-type of a Schema_A object type in Schema_B? Then you will also need to grant the "under" privilege on the object type to Schema_B. For example:
GRANT UNDER ON CUSTOMER_UPDATE_REC TO schema_b;
CREATE TYPE CUSTOMER_UPDATE_REC under Schema_A.CUSTOMER_UPDATE_REC ( ... );
Is this what you are after?