The concrete topping is not designed by the structural engineer and it is not needed to address a structural issue within the structural engineers scope so naturally the structural engineer would not specify the topping slab.
You either create a separate concrete specification section or you can negotiate changes to the structural engineers specification section that addresses your needs I suggest the former.
Remember the specification manual contains specifications from multiple consultants. Nothing prevents multiple consultants from collaborating in the writing of a specification section. Having said that when this is done there needs to be a record of which consultant was responsible for which provisions.