You can try this yourself if you've got Civil 3D and the latest version of InfraWorks. I opened one of the Civil 3D tutorial files, one that had a coordinate system assigned so that I knew how to georeference it. I dressed up the colors a bit and made it look good. The file is Parcel-1A if you want to try it yourself.
Next, I used the Raster Tools tab (Raster Design) to create a new image.
I used the default settings but you can tinker with the settings here to get the perfect resolution and image size. I used the Pick button to set the extents of the image to match the part of the drawing I want to show in InfraWorks. I also used the True Color option so that the colors of the image will match the colors I see on screen.
Now I have a blank image. I use the Merge Vector command to add the AutoCAD entities into my newly created image. Piece of cake: just pick the objects you want to include and tell the command whether you want to delete the original entities or keep them. I kept mine, there was no reason to delete them.
In the drawing, it may look like nothing has happened because a color copy of what you see on screen has been generated in raster form. Now it's time create a data source that InfraWorks will understand. I'll use the Image Export command on the Raster Tools ribbon.
For my example I used TIFF, Uncompressed, Tiled, with a World File (that's important for georeferencing). I took note that the coordinate system of my drawing is NH83.
Now it's time to go into InfraWorks. I created a model of the area using the Model Builder preview. How did I know where I was? I used the GEOMAP command to show me an aerial/roads hybrid in Civil 3D, noted some key roads, and found the same location in Model Builder. Then I added my file as a raster data source. I had to tell it the NH83 coordinate system, and then Voila! My AutoCAD drawing draped in my InfraWorks model.
So there you have it folks - an AutoCAD DWG shown in InfraWorks! What's that?....The black area?...Ah, we've got something for that too. I'll go back into the Data Source Configuration dialog to the Raster tab. There I find a Color Mask setting and when I click the ellipsis button I actually get a Pick Screen Color button. Using this I could tell InfraWorks to make the black transparent.
Oh, yeah...that's what I'm talkin' about! This is what I had in my mind when I thought about importing my AutoCAD drawing in InfraWorks.