you can make the preliminary model in SketchUp, export it to .obj (best to download the demo of SketchUp Pro, or if you are a rich snob, buy it), open it in Blender and fix everything, since MilkShape is iffy with sketchUp models if you don't know what you're doing.
Then, export it with Blender, (I can't explain how to install the dts exporter, since it's a pain) and open it in Torque Constructor (which is free). Save it as a .map, and then choose Export> .dif.
If all works, place the .dif in mission editor, and voila! Map!
...sadly, my saving in Constructor is broken :\
OMG I'm helping people >_>