Thank you, Michael!
I've already figured out.
I copied new phpexcel 1.8.0 in \library\phpexcel folder and added new line in kwf_lib_config.ini and in \kwf-lib\config.ini (old version commented by semicolon):
;externLibraryPath.phpexcel = %libraryPath%/phpexcel/1.7.6
externLibraryPath.phpexcel = %libraryPath%/phpexcel/1.8.0
It works like a charm now =)