Please define what you mean by "won't work".
The half-life engine reads map files under the extention of .bsp, hence there is no need to change them. If the file you have is not a bsp, then rename it to a .bsp for it to work.
If you mean the map isn't loading or showing up on your list, then you probably copied it to the wrong destination. The .bsp file goes into \esf\maps\ and the .tga files (if any) go into \esf\gfx\env\.
If there are any other problems regarding the map itself, then I would suggest you contact the S2k mapping team for further enquiries.
Hope you are able to sort this out, post back if you have trouble doing so.