Can I create my own financial categories and assign them to the Account segment?

Yes. You can add more categories to the list of Financial Categories on the Manage Values page.

In the Setup and Maintenance work area, use the Manage Standard Lookups task. Search for the FINANCIAL_CATEGORY lookup type and add a lookup code for each new financial category. To have the new financial categories appear in Oracle Transactional Business Intelligence reports the BI enabled option must be enabled for the Account segment.