To answer the question, no, there is no way to specifically mark a quiz as extra credit. For legacy quizzes, Canvas has stopped development on it and the best advice I've seen is in the thread: Creating an Extra Credit Quiz. There are several ways to accomplish extra credit listed there, including some that don't require any grading of the quiz itself.
I haven't used the New Quizzes outside of testing a few things, but it's an assignment that allows you to specify the points possible as well as the points for each question. It would seem that it would be easier to handle extra credit this way -- the quiz could have 10 points but the assignment is worth 0. Again, I do not use New Quizzes, so I cannot say for certain that this is how it behaves.
If you're talking about specific questions on a quiz, then there are ways to make the grading process quicker. I wrote a script that puts the fudge points at the top so you don't have to scroll all the way to the bottom to add them. It also adds a button next to the fudge points that will save the current score and advance to the next user so that you don't have click save, move your mouse to the top, click advance to the next user, and then repeat that process. Note that this doesn't make a question extra credit, it just speeds up the grading process.
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