Let's suppose you have a plan floor like this:
And you have imported your 2D geometry in Blender with your plugin:
Depending on what your plugin is capable of, you have to work a bit on the imported objects, maybe: merging, separing, removing doubles...
Than you can start extruding your 2D geometry. I suggest you to import also the elevation, if provided, in order to have quicker access on extruding values.
There are many ways in wich you can optimize the extrusion process (merge object, thickness modifier, curve extrusion,...) but all depend in which way your plugin get the 2D information out.
In my case I ony had the profile, so I had to create faces
and then extrude them:
continue till you are satisfied with your modeling by translating, extruding, adding objects.... with the help of the snaps.
Add a floor plane, a light, maybe textures and render...