@nativetec 

If by already paid payroll, you mean payroll paid before you subscribed to QBO Payroll, then there is an area in payroll setup where it asks you if you had other payroll in the year. You would answer Yes and it would have an area then to record previously paid payroll and payroll taxes paid.

If instead, you are doing after the fact payroll entry and you use a system outside QB for payroll, it's different.