This is few ways to approach, I am not sure which one would help you more, both options needs to tested according to your environment.
Option1:
Try the belopw statement in you DTS SOURCE
/* Need to rename the Col names */
SELECT COL1= case when COL1=NULL then 'XXX' end,
COL2= case when COL2=NULL then 'XXX' end,
COL3= case when COL3=NULL then 'XXX' end,
COL4= case when COL4=NULL then 'XXX' end,
COL5= case when COL5=NULL then 'XXX' end
from EXCELSHEET
Option2:
Use the Same DTS package
Step1:
From the Excel sheet Export into a temp table with 5 columns.
Step2:
Execute SQL statement and to update all Null values with your desired vlaue
Then Export to that data from temp table to your table
Or
INSERT INTO PRODTABLE
SELECT COL1= case when COL1=NULL then 'XXX' end,
COL2= case when COL2=NULL then 'XXX' end,
COL3= case when COL3=NULL then 'XXX' end,
COL4= case when COL4=NULL then 'XXX' end,
COL5= case when COL5=NULL then 'XXX' end
from TEMPTABLE
Let me know if you need any clarifications
Dr. Sql
goEdeveloper@yahoo.com
Good Luck.