SQL & PL/SQL

I want to place a null for values missing in the sub_account field. Here is my external table:

CREATE OR REPLACE DIRECTORY INCOMING_ORDERS_log_dir
AS 'c:\starpubs\starpubs\dataformats\logs\INCOMING_ORDERS\log';
CREATE OR REPLACE DIRECTORY INCOMING_ORDERS_bad_dir
AS 'c:\starpubs\starpubs\dataformats\logs\INCOMING_ORDERS\bad';
create table ext_INCOMING_ORDERS_table (
Account varchar(5),
Sub_Account varchar(1),
Override_Code varchar(1),
Nomenclature varchar(28),
chg_nbr varchar(3),
quantity integer,
U_I varchar(5),
zipcode varchar(5),
type_reject varchar(2)
)
organization external
(
type oracle_loader
default directory user_dir
access parameters
(
records delimited by newline
missing field values are null
badfile INCOMING_ORDERS_bad_dir:'INCOMING_ORDERS%a_%p.bad'
logfile INCOMING_ORDERS_log_dir:'INCOMING_ORDERS%a_%p.log'
fields
(
Account(1:5) char(5),
Sub_Account(7:7) char(1),
Override_Code(10:10) char(1),
Nomenclature(11:38) char(28),
chg_nbr(40:42) char(3),
quantity(44:48) integer external,
U_I(50:54) char(5),
zipcode(56:60) char(5),
type_reject(61:62) char(2)
)
)
location('PTCLICK.MANUAL.NOMEN.TXT','PTCLICK.ORDERS.TXT', 'EUR_RES.TXT', 'MQ.TXT', 'BPRO.TXT')
)
reject limit unlimited;


How can I place the MISSING FIELD VALUES ARE NULL for missing values for the sub_account?