There is no way to add a shift in the past since there is no way for an employee to punch in and out for a past shift. This also avoids issues with employees receiving their schedule via email or seeing it in the site and the shift not there until AFTER the shift happens.
If you need to add a time card for payroll, you can still do this via the Review, Update and Approve Time Cards page. Find the day and click ADD TIME CARD.
Note – If you called an employee in who wasn’t scheduled and they couldn’t punch in, take a look at setting them up with an OPEN SCHEDULE – https://support.timewellscheduled.com/article/open-schedule/