How can I enable all account segments for expense report users?

Set the value for the EXM_ALLOW_FULL_ACCT_OVERRIDE profile option to Yes. When you set the profile value to Yes, expense report users can change the segment values of expense accounts during expense entry to accurately reflect the charge allocations of their expenses.

You can set the value to Yes in the Account field on the Create Expense Item page. Navigate to: Setup and Maintenance > Search Tasks: Manage Administrator Profile Values > Search: EXM_ALLOW_FULL_ACCT_OVERRIDE.