That would take a lot of work noticing that the statsx.sma file has over 1700 lines. I think that a seperate Knife Rank plugin would be better because you wouldn't know who has the most knife kills in Top15. Try this it's much better:
http://forums.alliedmods.net/showthread.php?t=102413
the 2 versions in the thread are the same except the last version has no useless errors when compiling. if you plan on using the last version, you need to add "debug" after the plugin name in plugins.ini