How to implement Oracle label security in existing tables/procedures?

You can use the SA_POLICY_ADMIN package to apply policies to tables/procedures

SA_POLICY_ADMIN.APPLY_TABLE_POLICY( policy_name IN VARCHAR2, schema_name IN VARCHAR2, table_name IN VARCHAR2, table_options IN VARCHAR2 DEFAULT NULL, label_function IN VARCHAR2 DEFAULT NULL, predicate IN VARCHAR2 DEFAULT NULL);