What does the plan document say? It governs the plan, not the payroll company's system. If the document allows it and the Plan Administrator is the employer, then it is up to them to tell the payroll company what to do.
If the payroll company persists, ask them to contact their legal department to point out that by exercising discretionary authority over plan document interpretation and plan operations, they are making themselves an ERISA fiduciary to the plan. This may not get you anywhere, but it could be fun entertainment, at least if you're a pension geek like me.