Undo with spanElement

Hi

I have undo end it works perfect but when I add a span element it don't works:

var pElement:ParagraphFormattedElement = currentRange.firstParagraph;

var span:SpanElement = new SpanElement();
span.text = "ciaoo   ";
pElement.addChildAt(0,span);

I don't find operation to add spanElement on Editmaneger

http://livedocs.adobe.com/flex/gumbo/langref/

thanks