If the student has not started the exam
If you use Canvas' Moderate Quiz to extend the time before the student starts the Proctorio exam, the student will get a countdown timer that includes the full allotment of time. That is the time set in the Canvas settings plus that entered in Moderate Quiz > Extra time.
If the student has already started the exam
If you use Moderate Quiz to extend the amount of time after the student starts the Proctorio exam, then the countdown timer will be already running and it will use the time allotment set in the Canvas settings. The extended time will NOT be counted nor available to the student.
You may consider having the student quit that exam attempt and use Moderate Quiz to give the student another attempt. When they start the second attempt they will get the full time allotment of the Canvas settings plus the amount entered in Moderate Quiz > Extra time.
If your quiz is locked because of accessibility dates, you will need to unlock the quiz for the student by clicking the manually unlock the quiz for the next attempt checkbox.