load data
infile *

replace
into table test
( col_n integer external (10) "replace(:col_n, '?', ' ')" )

begindata
42
?
1889

SQL> create table test (col_n number(10));

Table created.

SQL> $sqlldr scott/tiger control=test.ctl log=test.log

SQL*Loader: Release 10.2.0.1.0 - Production on Sri Sij 2 21:07:04 2008

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Commit point reached - logical record count 2
Commit point reached - logical record count 3

SQL> select * from test;

     COL_N
----------
        42
      1889

SQL>