If you add an employee to a PTO Accrual rule, the rule will not automatically apply to previous pay periods.
To add accruals to a previous pay period you would need to visit Employees in the top navigation and select View next to the employees name. Once in their profile, select PTO on the left hand side. This will take you to the PTO section.
Once at the PTO section, select +Add PTO Accrual Entry. This is a one time entry that will allow you to add or subtract PTO from the selected PTO type.