even this is old topic, but this member still active
unfortunately not all stuffs can be turn into plugin, like adding custom cell_ type (setcell) ... etc..
but, plugin do provide function overload, which is kinda a double-edge-sword
you can overload the function with a plugin file, but you still have to update this plugin file periodically to prevent it from not working
also having 2 plugins overload the same function, defeats the purpose of using plugin
so for your answer, there is no other choice but to copy paste the skill_additional_effect function into the plugin, with the lines you wish to edit
https://github.com/HerculesWS/Hercules/wiki/Hercules-Plugin-Manager#hpm-function-overloading