Can I add time to a quiz for certain students?

YES! Canvas allows you to ""Moderate"" quizzes, which allows you to add time or additional attempts for certain users. Here's how you can do it:
Open Quizzes
Click on the Quizzes link.
Locate Quiz Link
Find the quiz you want to moderate and click on the quiz link.
Locate Moderate This Quiz
Once you click on the quiz link, you will need to then click on the Moderate This Quiz button.
Moderate Quiz

Here you can moderate if you want to give a student another attempt or more time on a quiz by clicking on the Pencil icon by the student's name.

Moderate Quiz For Multiple Students
Here you can moderate if you want to give multiple students more attempts or more time on a quiz by clicking on the checkbox next to the names of the students or if you want to select all, click the top checkbox. Click Change Extensions for 3 Selected Students to give extensions to the students.

Change User Extensions
When you click on the pencil icon, you will see a pop out window entitled Student Extensions. Here you can add Extra Attempts [1] and Extra Time on Every Attempt [2], as well as Manually unlock the quiz for the next attempt [3] if you want to make sure the student can re-take the quiz. Be sure to click Save before exiting the window.

View Extensions
Once you click Save, you will see the extension changes you made for the student(s). For Agent 99, she gets extra minutes on each attempt [1], has 2 attempts left [2], and has the quiz manually unlocked [3].

Add Extra Attempt
Another way to give a student an extra attempt after making extension changes is to click on the student's name link and find the allow this student an extra attempt button. You can automatically give a student an extra attempt for the quiz.