To be precise, you need to adhere to the Recruitment/Selection/Appointment policy of the company. If the probation confirmation period is stated to be six months, allow the employee to work up to six months and then confirm their services.
However, if your company management is very particular about reducing the probation period and confirming the employee, your HR Department can issue a letter of confirmation, citing the reason for their superior performance in their current role. Please ensure that necessary approvals are obtained from top management (MD, Director, or CEO) for any deviations from the policy. Properly document the same and keep it in the employee's personal file.