I teach Pre Calculus and I have not found a way to have students enter complex numbers. If you choose to make it multiple choice you can use the equation editor to add complex numbers in the choices, but I do not think there is a way to have students type complex numbers in a fill in the blank type question. (It would be nice if it was possible though)
I was also looking at the formula and numeric questions in new Canvas quizzes. Can complex numbers be used at all in new/old quizzes? I wasn't able to find any information on this. Thanks.