This is a two-step process. First, you must create the exam assignment in Canvas for a column to appear in your Gradebook. Then, you will be able to enter grades in a newly created grade column.
Go to Assignments > +Assignment
Enter and choose the assignment name and settings
For Submission Type, choose No Submission
Click Save and Publish
Go to Grades to view your Gradebook and the new exam column
Enter the score manually in each cell for every student
