SS3 - Bottom Mesh from Surface Template

I'm working on a site model for an outdoor track and field complex.  So far, I've completed my horizontal and vertical geometry, created terrain objects (one for the track, another for the sidewalk, a third for the infield, and a fourth for the outfield), and assigned surface templates to each.

I'm trying to do some earthwork balance calculations using the SS3 tools. My plan was to change the design stage to add bottom meshes, then create a composite terrain from those to compare with my existing terrain.  When I navigated to the Project Settings, however, it appears that there are only design stages for "Corridors", "Linear Templates", and "Survey".  I changed the design stage for the Linear Templates to add bottom meshes, but it didn't seem to have any effect.

Is there any way to get a bottom mesh from surface templates?  If not, I obviously should have built my model differently.  Any suggestions would be greatly appreciated.