You can easily add a bank account for an existing customer by logging in to MyCodaBox.

Search and Select your customer in the CLIENTS - Client List menu.

In the details screen of your customer you can add a bank account by clicking on New Coda

Mandate in the Mandates section:

In the next screen, enter the bank account(s) you want to add:

The registered bank accounts can have different states, these are described in our article Bank accounts, what do the different states mean?

See also our article Possible situations when adding a bank account.