Userexit For VA02

Hii..

I have to enhance the VA02 transaction.. the requirement is.. when the user gives the sales order.. so after pressing enter..

the next screen.. should come with two line items.. if it is having one line item.. the second line item should come automatically.. with the same.. row as it is in the original.. just.. two rows should display.. after pressing enter..

So, i am using.. USEREXIT_FIELD_MODIFICATION for doing this.. and i m updating the xvbap[] table also..

but.. it is not displaying in the output..

Do anyone.. help this..

Regards,

Jhings