Processing Question if to create a logical or else?

The file that has the data I need is keyed as follows:
  K CO 
  K ITEM  
  K CENT 
  K YEAR
The monthly summaries of items shipped is in this file in an array form. 

The program to be modified is printing summaries by month/year on the carton 
level. In order to get total # of items shipped, I feel that I need a logical 
that will key CO & Year, because of the existing key structure I cannot do a 
SETLL on CO & Item, bec. I don't have the item in the existing program. I would 
then create the total items needed based on the dates entered by the user.

Is there another way of doing this, or is creating the logical the really best 
way?

Thanks,
Rob