(*) required a FCKEditor 2.6 available on / apache path (means /fckeditor points to fckeditor root directory)
add custom config like this code :
var oFCKeditor = new FCKeditor( 'rssTest' ) ;
oFCKeditor.Config["CustomConfigurationsPath"] = oFCKeditor.BasePath+"/myfckconfig.js";
oFCKeditor.Create() ;