ERPNext allows you to add additional Salary Components (Is Additional Component) to Salary Slips while processing the payroll. Some examples of Additional Salary could be Performance Bonus, Deputation Allowance, Arrears or other adjustments etc. To create Additional Salary, you can go to -
Human resources > Payroll > Additional Salary > New Additional Salary
You can select the Employee to whom you wish to disburse the Additional Salary, the Salary Component and the period for which the employee is entitled to receive the Additional Salary. If the period (From and To dates) consists on multiple payroll periods, the amount will be disbursed on a pro-rata basis.
Note: The Salary Component can also be a deduction component which will add the Additional Salary to the Deductions table in the Salary Slip