Valve probably wont care, but afaik this violates many terms of service. In taking an asset from CZ and putting it in a mod for HL, you are allowing people who do not own CZ to access and use materials from a game they do not own. In a sense, you are distributing copyrighted material (albeit very small amounts) to people who may not own the game.
I've had to deal with this matter before. I personally dont care one way or another, I'm just telling you how lawyers see it (US lawyers, that is). Valve most likely wont care, but there are other publishers/devs that are much more anal about this sort of thing.