What is the most suitable option for a massive upload on Invoices in Payables?

Summary:

Customer has more than 2000 AP Invoices per month. As per legal requirements for Tax Authorities reporting, there are several fields that must be manually populated.

Content (required):

Customer wants to avoid the manual creation invoices with lof of fields to be manually populated. They want to get a suitable method for a massive upload of the invoices with all the required fields already populated. We're trying to create a spreadsheet in FBDI for a massive upload of the data, but we're having problems because values for those fields are refused by the system. Question is: do you know any other way for a massive upload of the invoices to be imported with these all data populated ?