how about using arbitray column names in the control file for non existing columns in table?
--
-- this table has only 3 columns
--

mag@mutation_mutation > desc filler;
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 COL1                                                           VARCHAR2(5)
 COL2                                                           VARCHAR2(5)
 COL3                                                           VARCHAR2(5)

mag@mutation_mutation > select * from filler;

no rows selected

mag@mutation_mutation > exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

--
-- this is the data file
-- it contains 10 fields.
-- and we want to load only 1,2 and 5 fields into col1,col2,col3
--
--

bash-2.03$ cat filler.dat
123,246,333,121,121,121,24d,zzz,777,xxx
123,246,333,121,121,121,24d,zzz,777,xxx
123,246,333,121,121,121,24d,zzz,yyy,xxx
123,246,333,121,121,121,24d,zzz,yyy,xxx

--
-- this is the control file
-- create dummy columns to match the number of fields in data file.
-- use original column names ( as in table) for the fields u want to load
-- for the rest dummy column names ( which dont exist in the table) use FILLER
--
bash-2.03$ cat filler.ctl
load data
infile 'filler.dat'
insert into table filler
fields terminated by ',' trailing nullcols
(
col1 ,
col2 ,
dummy_col3 FILLER,
dummy_col4 FILLER,
col3 ,
dummy_col6 FILLER,
dummy_col7 FILLER,
dummy_col8 FILLER,
dummy_col9 FILLER,
dummy_col10 FILLER
)

--
-- Lets load this and check
--

bash-2.03$ sqlldr userid=mag/mag control=filler.ctl

SQL*Loader: Release 9.2.0.1.0 - Production on Sat Feb 7 09:28:20 2004

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Commit point reached - logical record count 4
bash-2.03$ sqlplus mag/mag

SQL*Plus: Release 9.2.0.1.0 - Production on Sat Feb 7 09:28:25 2004

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

mag@mutation_mutation > select * from filler;

COL1  COL2  COL3
----- ----- -----
123   246   121
123   246   121
123   246   121
123   246   121