Navigate to Payroll Admin>Payroll Setup>Import csv records
The import type would be "custom record" and the record type is "pay component"
1. The start date of the pay component needs to be the start date of the next payroll period you are running.
2. The back pay to date needs to be able to be compared with the start date to determine the amount of the back pay.
3. When you do the import mapping options you need to ensure:
a. It is run as an "add" not update
b. You use the "Salary/Wages Pay component form"
c. "Run server script and trigger workflows" is ticked.