Sub account numbers for banking

We use BMO as one of our banks. They have the bank accounts setup where there is a master account number that you use on their website and on the BAI file. Then there is a subaccount number that is used on the checks and positive pay file. We are exploring how we should set this up. The users would prefer that we list the master bank account number on the external account and for reconciliation, but how can we get a different bank account to show up on the check and positive pay (preferably without customization)? If we put