If you are using export from grid from Account Ledger Inquiry (P09200), you can only export one account (BU.OBJ.SUB) at a time. If you want the same object account for multiple business units, try Account Inquiry by Object Account (P09201). If you need random accounts and are willing to assign the same account (F0901) category code value to all of them, try Account Inquiry by Category Code (P09202).
If you have access to Report Design Aid/Object Management Workbench, you could write a custom report over F0911 which outputs to CSV instead of PDF and use data selection to choose the accounts you want to export.
Please let me know if you need more information about P09202 and if any of these methods meet your needs.
Brad