how to extend the budgetary control functionality in Oracle Internet Expense

Hi

We're considering to use budgetary control to monitor the expense spending. We assume that if your expense report amount exceed the available year budget, then the system should block the submit progress till the budget is available. But refer to Oracle Internet Expense user guide, I did not find any solution to support this functionality.

Hence we need to customize the oracle standard page to extend this functionality. Should  we customize oracle standard FinalReviewPageCO and WebExpensesAMImpl or oracle has some other packages will support customization extension? I know that oracle provides AP_WEB_OA_CUSTOM_PKG to extend the message. Thanks