I can insert my custom css inside editor's document/head object like
<telerik:radeditor runat="server" ID="RadEditor1" >
<telerik:EditorCssFile Value="mycss.css" />
I have developed some custom tools, like my own image and when user mouseovers the image in editor I want to display some alignment buttons or information icons on it. So I have written the code on mouseover-out etc events in my custom js file.