Pre Tax Retirement Plans

My company is rolling out a new retirement plan for our execs and I am having an interesting time figuring out how to set this up in the system.

We are rolling out a non-qualified deferred compensation plan, which is calculated on a pre-tax basis. The contribution to this non-qualified plan should then be subtracted from the gross wages before the 401K deduction is calculated and taken in the system. I feel like this may not be an option, since both the gross wages and deductions are calculated during the pre-payroll. If anyone has any ideas, I would love to hear them!