@JeffSchwenk I think the first thing needed is to get the customer to define the purpose of the spreadsheet.
The process you are describing is not unusual. A voucher is created and then routed through the office for approval prior to the invoice being entered into the accounting system. If this is the case, I suggest discussing doclink or some other document management tool that will avoid double entry and improve workflow processing.
If they are just doing this because they always have and no one is approving the voucher prior to entry, put a page break by invoice in the invoice register so you get one invoice per page and use that as your voucher.
If they are truly wed to their Excel entry process, create a VI job to import the invoices and then write a macro in Excel to take the data in excel, copy it to another worksheet in the workbook in a format necessary for import, and then call the VI job to import the info to save them from re-entering.
@Wayne, I don't want to hear any TL/DR comments! ;-)