The employee needs to have in place an agreement with the Employer to salary sacrifice their termination payments. For more details, refer to the Australian Taxation Office (ATO). The links below are a starting point:
You will need to process the termination as a manual calculation, as deductions before tax can't be included in a termination transaction.
If part of the termination is an ETP, then this cannot be salary sacrificed to superannuation. Seek financial advice first.