As suggested by Jamie, Its not possible with Transformations since your schema keeps on chaning so the underlying metadata will throws error.
You have to think of some other workarunds like,
Using alter script add a field when new one needed.
Do the field mapping using stored procedure.
Always I used to think why the ETL flow in SSIS is so much bounded with the metadata that's defined at design time..
I dont know how to achieve this with the existing transformations.. please help
Like I said, its not possible with the existing transformations. You will need to import all the columns that you want to unpivot as a single column (explore the Ragged Right feature of the Flat File Source Adapter) and then write your unpivoting logic inside a script component or (if you're brave) a custom component.
http://sqlblog.com/blogs/jamie_thomson/ | @jamiet | About me