3ds Max Modeling :: Merging Hollow Meshes For 3D Printing

This is part of a project to 3D print models of cities using data extracted from public sources.  I have extracted the model data for cities, but cannot get models that will print correctly.  From the attached file, you will see that I created a simple standard primitive rectangular base, but there are also sorts of holes in the meshes of the cities. Do you 3DS wizards know any way to :
 
1) combine the solid rectangular primitive so that it will be solid through the meshes that it intersects; and/or 
 
2) fill the sometimes hollow 3d building models so that I can print them (regardless of the interiors).