I found this solution on a forum, but somewhy this forum was not accessible the second time i tried to read it, as it insisted that i registered. It was still avaible trough google cache, but to make sure information is free, i will publish the solution here as well.

So you probably have wanted to play custom maps in Call of Duty 4 without having to run a mod (who usually doesnt have the possibility to make and save classes). A CoD4 custom map-package usually contains three files: a *.iwd file and two *.ff files.

1. Put both your *.ff files to the folder "[Call Of Duty Directory]/zone/english/".
2. And put your *.iwd file into the "[Call Of Duty Directory]/main/".
3. Rename the *.iwd file you just copied to "iw_XX.iwd", where the XX is a number one higher than the highest number of the iw_ files in this folder.

I have not tested how many such maps you can add. If you ever hit a limit, extract all of the *.iwd files as if they were a zip package (rename them to .zip, and extract them), then package all the files that was extracted into a new package, which you afterwards rename to "iw_XX.iwd". My *.iwd file ended up as a friggin 2 gb. file.

As a last note: All of your players who want to play, have to have both the *.iwd files (with the same numbers) and the *.ff files on their computer as well.
Good gaming!