create or replace function get_key 
  (p_col in lookup_table.col1%type)
  return    lookup_table.col2%type
as
  v_result  lookup_table.col2%type;
begin
  select col2
  into   v_result
  from   lookup_table 
  where  col1 = p_col;
  return v_result;
end get_key;
/

load data
infile 'data.txt'
into table dwh_table
fields terminated by ','
trailing nullcols
(col1,
col2 "get_key (:col1)")

col1;col2;col3;col4
value1;value2;value3;value4

LOAD DATA INFILE 'abc.txt' INTO TABLE ABC
APPEND
FIELDS TERMINATED BY ';'
TRAILING NULLCOLS
(
COLUMN1,
column2,
column3,
COLUMN4,
COLUMN5 ":column2 || :column3"
)

scott@ORA92> create table abc
  2    (column1 varchar2(10),
  3  	column4 varchar2(10),
  4  	column5 varchar2(20))
  5  /

Table created.

scott@ORA92> alter table abc add (column2 varchar2(10), column3 varchar2(10))
  2  /

Table altered.

scott@ORA92> host sqlldr scott/tiger control=test.ctl log=test.log

scott@ORA92> alter table abc drop column column2
  2  /

Table altered.

scott@ORA92> alter table abc drop column column3
  2  /

Table altered.

scott@ORA92> select * from abc
  2  /

COLUMN1    COLUMN4    COLUMN5
---------- ---------- --------------------
col1       col4       col2col3
value1     value4     value2value3

scott@ORA92>