They key is to access the document object of the CKE iframe. Then you just vomit a CSS link to the head of the document without touching the config. Example is from https://stackoverflow.com/a/577002/694325
I am assuming that you use "editor1" for the name, but use whatever you have..
var doc = CKEDITOR.instances.editor1.document.$; // get CKE doc!
var cssId = 'myCss';
if (!doc.getElementById(cssId))
{
var head = doc.getElementsByTagName('head')[0];
var link = doc.createElement('link');
link.id = cssId;
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = 'http://my.little.pony.net/Your.css';
link.media = 'all';
head.appendChild(link);
}
Or you could jQuery it if you are into that kind of stuff (https://stackoverflow.com/a/2685661/694325)
var doc = CKEDITOR.instances.editor1.document.$; // shortcut
$("<link/>", {
rel: "stylesheet",
type: "text/css",
href: "http://my.little.pony.net/Your.css"
}).appendTo($(doc).find("head"));