The customer query subject in Cognos doesn't have the information which billing office a customer is assigned to. The reason is that a customer can be associated to multiple billing offices, hence it's not possible to add a single field into the customer.
In order to facilitate reporting on the customer billing offices, Changepoint provides the view "CustomerBillingOfficeNames". It shows for each customer a concatenated list of all associated billing offices in a text field.
To use this in Cognos, define a query based on this view:
select c.name, c.customerid, convert(nvarchar(2000), cb.BillingOfficeNames) BillingOfficeNames from customer c left outer join CustomerBillingOfficeNames cb on cb.customerid=c.customerid where c.deleted=0
Then join this query via customerid to your existing report.