Try this

If you want in jquery then try this

$(document).ready(function(){

   var clickfun = $(#button1).attr(onClick);

   var funname = clickfun.substring(0,clickfun.indexOf(());       

   $(#button1).attr(onclick,funname+('somethingDiff1',+'somethingDiff2'));

  });

<script src=https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js></script>

<button id=button1 onClick=someFunc('arg1','arg2')>Button1</button> 

If you want in JavaScript then try this:-

<script>

  window.onload=function(){

  var clickfun = document.getElementById(button1).getAttribute(onclick);

  var funname = clickfun.substring(0,clickfun.indexOf(());       

  document.getElementById(button1).setAttribute(onclick,funname+('somethingDiff1',+'somethingDiff2'));    

};

</script>

Hope This will help you.