If you needing to convert some existing models , you might want to take a peek at Blender, and the Valve supported exporters for HalfLife 2.
https://developer.valvesoftware.com/...nder_SMD_Tools
You can import and export everything except .MAX files with blender. Most people will export OBJ, or 3DS files for static meshes, and Collada or FBX for animated things from 3D studio, to get them into Blender.
Blender makes it easy to use Unity too. Unity can read .Blend files, and automatically convert everything over.
Hope you find a solution, those models look really nice!
EDIT:
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EDIT EDIT: Fixed link, i was linking to old page:
https://developer.valvesoftware.com/...nder_SMD_Tools