Hi Amirthraj,
If the flat file is static (if the content within it is same for all) then use the offset concept. First get the contents of the flat file into an internal table and later find the field that has to be in final internal table within the line, calculate the offset for the line and get the value into an workarea. Finally modify/append the work area into an internal table. Also wait for other experts answers for any other hassle free way to achieve the requirement.
With Regards,
Giriesh M