You can add grading periods at any time, thought it is best to do so when you first create a course. These instructions detail how to add a grading period after the course has been created.
First, navigate to the course you wish to add a grading period to. Then click on the Gradebook tab.
This will open the Gradebook, from here, click on the Settings icon, then choose Setup.
To add a grading period, scroll to the bottom of the page and click Add Category.
Type the Name of your grading period in the Category name box, then Check the box next to Grading Period.
Click Save Changes to return to your course. You will now see your grading periods appear in your gradebook and whenever you add an activity you can associate it with a grading period.
For additional support, contact firstname.lastname@example.org