The Blackboard grade center allows instructors to create a column that is not tied to a quiz or assignment. In Canvas instructors cannot create columns directly in the grade book. All columns are generated by the assignments and quizzes in the course. To get the same functionality as a manually created column in Blackboard, you must create an Assignment in Canvas with the Submission Type option set to No Submission.
Following is a link to a help article in the Canvas Guides on how to create an assignment. Scroll down to the Submission Type option to learn more about No Submission assignments.
Learn how to Create an Assignment and Set Assignment Details.