Pension Override

If there's no pensionable pay in current month, but employee would like to make a one-off contribution on top of his usual contribution rate, usually, the employee contribution will not appear in the payroll screen. So, we are forced to override the pension from Employee screen, but this will remain fixed and is risky. Is there any other way to process it? Why can’t we add pension contribution pay code manually to payroll screen to make a one off contribution?