Sorry, I don't know of any way to do that.
In Hammer, everything is made of solid blocks of material called "brushes". Your 3ds files probably don't work anything like that.
You'd have to have a pretty specialized program for converting from one thing to the other.
A big problem is that in the game (HL, CS, etc) the map is in BSP format. You can't just build whatever you want. It has to be compatible with BSP. The simple way I can explain this is that your map has to be made of very simple rooms and hallways.
You can't even have big expansive areas (outside) without breaking the game. Go read about BSP to determine if you think your data is even compatible with this format: