Hi James,
listing / exclusion uses the condition technique. So one approach could be to set up the access sequence using KVGR1 or any other field from the customer master that links the WalMart customers.
I suppose at the moment you use the customer number in the access sequence? Check if it would be ok to change that to e.g. KVGR1.
Please also check the USEREXIT_MOVE_FIELD_TO_KOMPG (MV45AFZA) and related notes:
863767 - FAQ: Listing/exclusion (question 9)
Regards
Patrick