Yes, you can add the volunteer hours manually in Salesforce if the volunteer has forgotten to Check-in for a Jobs/Shift.

You should collect all the information from the volunteer, like Job, Shift ID, Check-In and Check-Out time and then enter those details manually from the Timesheet details screen in Salesforce.

The timesheet details are now updated for the volunteer who forgot to Check-In. These details are automatically updated to the related Volunteer Hours records.