No, it is not possible to modify the Flat File Bill's format. You can, however, use the Flat File Bill Tool to analyze the file data. If the pre-defined reports do not meet your needs, you can develop custom queries and reports with the Flat File Bill tool.
UPS offers UPS Billing Data and its companion Billing Analysis Tool as the next generation of the Flat File Bill and Flat File Bill Tool. Both solutions provide a way for you to integrate electronic billing information, along with a tool for viewing and analyzing billing data.