As you say, your customer has multiple sales area, itself means, you are trying to load Customer Sales View. SAP BI provides separate business content infoobject called 0cust_sales and datasource 0cust_sales_attr. You should try to enhance this datasource with other required fields.
If customer has multiple partner functions, then better to add additional attributes in 0cust_sales and then write code in CMOD in R/3 to enhance the extractor to populate those attributes.
E.g. different attributes for Ship to , Payer, Bill to etc.
Regards
Saurabh