Yes, that's possible.

 

Here is an example:

 

<CE:Editor id="Editor1" runat="server" TemplateItemList="PasteWord, insertcustombutonhere"></CE:Editor>


 void Page_Load(object sender, System.EventArgs e)
 {
  CuteEditor.ToolControl tc = Editor1.ToolControls["insertcustombutonhere"];
  if(tc!=null)
  {    
   System.Web.UI.WebControls.Image Image1 = new System.Web.UI.WebControls.Image ();
   Image1.ToolTip    = "Insert today's date";
   Image1.ImageUrl    = "/images/PasteWord.gif";
   Image1.CssClass    = "CuteEditorButton";
   SetMouseEvents(Image1);
   Image1.Attributes["onclick"]="CuteEditor_GetEditor(this).ExecCommand('PasteWord')";
  }
 }
 
 void SetMouseEvents(WebControl control)
 {
  control.Attributes["onmouseover"]="CuteEditor_ButtonCommandOver(this)";
  control.Attributes["onmouseout"]="CuteEditor_ButtonCommandOut(this)";
  control.Attributes["onmousedown"]="CuteEditor_ButtonCommandDown(this)";
  control.Attributes["onmouseup"]="CuteEditor_ButtonCommandUp(this)";
  control.Attributes["ondragstart"]="CuteEditor_CancelEvent()";
 }