This way would work for Games like Halo, which are using a §D App. for level editing.
Hammer is as different as it can get from this method.
In hammer you build your maps out of basic geomety brushes like Cubes.
The map area itself has to be sealed off completly from the "void" arround it.
The bsp compiling will delete every face which cannot be seen by a entity in the map and create the final polygon struckture.
So in theory your map could work under these conditions:
1. there cannot be any 2D elements, no single polygons, no planes, everything has to consist of solid geometry.
2. There cannot be any invalid struckture like concave faces
3. everything has to be aligned to the grid.
4. i'm sure there are a few more conditions,
- maybe material asignment
- smoothing groops, i guess the same rules as for Collision meshes
As you might have noticed now, it's not exactly easy to create a map for Source outside of hammer.
Good luck if you wanna try it, but i think the best decission would be, to build your map in hammer, using the 3D data you got as reference.
You could try to build a very basic version of the map in your 3D app. and export it to VMF.
Use the file in Hammer for size reference and do all the detail work there.
TBH, Dam isn't exactly the easiest map to start with. It has many things in it which Hammer doesn't like very much.
Another way might be to compile the model as a prop enable the "show models in 2D" button in hammer, so the wireframe of the model will be visible in the 2 viewports of hammer and you got some guidelines to build your model from.