3DSMax. No really!
What you do is open Hammer, load up your map.
Click FILE then "Export to DXF".
You now have a format you can load up in 3DSMax, since 3DSMax will read DXF files.
All you have to do from there is use 3DSMax to export that geometry to the SMD format used for raw HL models.
What hammer version? Me wants to try that!
In the mean while you could use
Deep Exploration Right Hemisphere - Visual Product Communication and Collaboration.
Texture will be missed though.
GL. ;D