SQL*Loader Control File Syntax

Hello everyone! I'm new to Oracle and I'm in the middle of transferring our data from DataEase into Oracle 8i using SQL*Loader. I want to enter a 1 where INVCUST='yes' and a 0 where INVCUST='no'. I'm having trouble with the proper syntax. I've included a little example of one way I've tried and it's not working for me. Would anyone be able to tell me what I'm doing wrong?
Thanks!
Lesley

FBEImp.ctl ........

Load Data

INFILE 'fbe.txt' "var"
BADFILE 'fbe.bad'
DISCARDFILE 'fbe.dsc'

INSERT INTO TABLE frghtbill
fields terminated by '~' optionally enclosed by " "

TRAILING NULLCOLS
(
....

FIELDS
INVCUST iif(INVCUST='yes',1,0),
FIELDS
....

terminated by whitespace)

I've also tried
INVCUST IF INVCUST='yes' THEN

                            INVCUST=1;
                     ELSE
                            INVCUST=0;
                     END IF