Hi yoshu,
Since you mistakenly terminated the employee, you can indeed reverse the termination. Here's a helpful link to assist you with this, Reinstating an employee. After reversing the termination, update the employee's status to "Casual" in their employment details. Since they're now casual, review any entitlement categories (such as leave) to ensure they're appropriate for a casual employee. Also, check with your super clearing house to confirm if any adjustments are required for superannuation contributions, though typically, this change shouldn't affect super payments.
This adjustment won't create two payment summaries as longs as you don't create a new employee card. The existing record will consolidate all payments for the financial year.
Regards,
Sai