Requirements: Basic knowledge in Radiant, Half-Life/Counter-Strike, patience.
Note: This will not work for Source Engine maps (i'm trying to find a way to make it happen though).
Firstly, you'll need to download WinBSPC to decompile the map, HL Texture Tools to extract the textures, an image manipulation software to convert the textures to DDS and GDT Generator to generate the GDT file for the asset manager.
WinBSPC: [You are not allowed to view external links. Register or Login to see them]
HL Texture Tools: [You are not allowed to view external links. Register or Login to see them]
GDT Generator: [You are not allowed to view external links. Register or Login to see them]
To decompile the map, run WinBSPC, go to File -> Convert.
Find the bsp file of the map you want to decompile. (bsp files are in root/mod/maps/ [root: half-life's root folder, mod: the folder of the mod, cstrike for instance])
Check MAP and choose the output folder and press OK.
Now you can open the file in Radiant. If you followed the previous steps correctly, Radiant will be able to open the file and it'll look like this.
No need to worry, since you haven't exported the textures and converted for CoDWaW yet, it's normal for it to look like this. You'll need to get the names of the textures from the textures window.
Before running HL Texture tools, you'll need to get the names of the .wad files which have texture files in them. In order to do it, open the .map file with a text editor, you'll see something like this.
You'll need to open those .wad files on HL Texture Tools. In order to do it, run HL Texture Tools, go to File -> Open and find the .wad files written in .map file one by one (except decals.wad, you won't need it). After opening the .wad file, go to File -> Extract all and select the choose the folder you want to extract the textures into.
After extracting the textures, you'll need to learn how to convert them for waw, but i won't explain how to make custom textures since there are tons of tutorials on how to do it (i.e [You are not allowed to view external links. Register or Login to see them] )
If you don't want to add the textures on asset manager one by one, I recommend you use the GDT Generator. There is a readme file that explains how to use it, but you'll need to convert transparent textures like fences manually since GDT Generator doesn't support that for now (I'll try to make a better texture tool to make texture converting easier, but for now, we'll have use this).
Note: I recommend you generate the GDT files seperately for different kind of textures like metal and wood.
After generating the GDT file and converting the textures, you can open the map on Radiant again. The textures will look a bit rubbish, you'll need to use surface inspector on Radiant to fix them. In order to do it, select the face of the brush by pressing Shift+Left Click and press S, then set vertical and horizonntal stretch to 1 (it works on most of them, but sometimes you need to change those values). If it's a crate or a box, you don't need to use surface inspector. Just select the whole box and press CTRL + F to make the texture fit.
After fixing the textures you'll need to delete the brushes that look like this since they are triggers and stuff and you won't need them. You'll need to delete the ladders and make the laders on your own because the ladder textures of Half-Life looks ugly in cod.
Lastly, you'll need to delete the unnecessary entities like these.
After completing these steps you can safely edit the map however you want and add stuff related to zombie mode.
Personal request: It took me lots of time to figure out how to do this in the first place so I'll be glad if you give me credit if you remake a map using this tutorial. Also, cs_assault has been already remade by me and de_dust2 is being remade by Ray1235, but he hasn't finished it yet. Please consider it before choosing the map you're gonna remake.
Feel free to reply to this topic if you have any problems. I'll try to do my best to help you.