Aldo,
use the following method:
//about Italic, see Full.config
//<item type="image" name="Italic" imagename="Italic" />
//get the pos after the Italic
int pos=Editor1.ToolControls.IndexOf("Italic")+1;
//Themes/%ThemeName%/Images/text.gif
WebControl ctrl=Editor1.CreateCommandButton("MyButton","text.gif","Insert My Custom Text");
ctrl.Attributes["onclick"]="ShowMyDialog(this)";
//add this custom button into the editor
Editor1.InsertToolControl(pos,"MyButton",ctrl);
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