Any flat fee work can have additional hourly billing. When entering time, you set "Included in the Fixed Fee" to no. (see image one). This designates this time entry to be out of the scope of the fixed fee and billed hourly at your designated rate.
When it comes time to invoice, you can choose what to include; fixed fee, expenses, hourly billing or the time that went into the fixed fee item.
Image One: Fixed Fee Hourly Billing.