A custom button (Inert the current time)

System.Web.UI.WebControls.Image Image1 = new System.Web.UI.WebControls.Image ();
Image1.ToolTip = "Insert the current time";
Image1.ImageUrl = HttpContext.Current.Request.ApplicationPath +"/timer.gif";
Image1.CssClass = "button";
Image1.Attributes.Add("onclick","var d = new Date(); Paste_Html('Editor1',d.toLocaleTimeString())"); Image1.Attributes.Add("type","btn");

Add your custom button at a specific location

 Editor1.toolbar.Add(Editor1.Bold);
 Editor1.toolbar.Add(Editor1.Italic);
 Editor1.toolbar.Add(Image1);   Add your custom button here
 Editor1.toolbar.Add(Editor1.Underline);  
 Editor1.toolbar.Add(Editor1.Separator);

Put the rest of buttons to the Template property.

<CE:Editor id="Editor1" runat="server" Template="JustifyLeft,JustifyCenter,JustifyRight,Separator,FontColor,HighLight"></CE:Editor>