I would like to know, if anyone knows, how to import L4D2 stuff and make it usable on hammer for a portal 2 map. I would like to use the weather effect from the map hard rain, and make it playable on my custom map of Portal 2. Is it really hard to do ? Thx for your help.

Maybe it's not hard but I think you shouldn't do that because it's content from another game and you are not the owner of copyright for that stuff.