Once more to make sure you understand:
In order to update the hullfile the map would have to be recompiled with the original .map file as a source. Since you don't have that .map file you're in tough luck.
Theoretically it should be possible to create a very complex program that somehow locates the fourth clipping hull, uses it with a new hullsize to create a third clipping hull and have that one overwrite the original third. Unfortunately, even if this very complex operation is finished it would most likely break the BSP tree and cause even more trouble.
Also, as you may have noticed, XP-Cagey has been employed in the industry and is no longer allowed to work outside his current company, even if it's for free.
You could also decompile the CS map, just take a look at
this 117 page thread to give you an idea of the amount of work involved...
In short: Not Gonna Happen
tm.