Need help with SQL SET statement

["Shaheen Ahmed" <Syed.Ahmed@xxxxxxxxxxxxxxx>]

I need to update a 12 characters field starting from position 6 to 9 with '0000' using interactive SQL. 
First I need to check if the 12 character field starting from position 6 to 12 is blank then update this field from start position 6 with 0000.
 
Before MYFLD = '12345          '
After   MYFLD = '123450000   '
 
Something like this 
 
 UPDATE MYLIB/MFFILE  SET myFLD = substr(myFLD,6,4) ='0000'
 where myFLD = substr(myFLD,6,8) ='    '                         
 
TIA