Adding Floor Structure Layers

[Marc_D]

I am using Home designer Pro 10.

I am working on a plan in which a cistern will be a part of the foundation. Rain water catchement is the best water supply option in this location.

I am unable to modify the ceiling characteristics for the cistern or the floor defaults for the rooms above to include extra layers between the cistern and the living areas above. The layers will be (from top to bottom) Finished flooring, sub-floor, Sleepers (within rigid insulation between sleepers), Vapor barrier, Concrete Cistern lid (including all the re-bar within the lid).
All I can find in the room specifications dialogue/stucture/floor structure are 2 layers.