Ok ,it truned out very easy but I will post an answer here in case someone is still searching for this ,
1- Map flat elements between excel cells with XML Map source .
2- VBA code to export the XML :
    'Export XMLMap from worksheet,this will export the flat Mapped elements
    Set Map = ActiveWorkbook.XmlMaps(1)
    Map.Export Url:=XmlFile, Overwrite:=True
    're-Load XML as DOM to process it
    Set XDoc = New MSXML2.DOMDocument
    XDoc.async = False: XDoc.validateOnParse = False
    XDoc.Load XmlFile
    'Update  XML with nested elements
    CellText = sheet_Board.Cells(22, "C").Value 'cell having nested elements values separated with "," ===> AAA,BBB,CCC
    Set Node = XDoc.SelectSingleNode("//Block")
    Set nodChild = XDoc.createElement("Nested_Elements")
    Result = Split(CellText, ",")
    For i = LBound(Result()) To UBound(Result())
        Set nodGrandChild = XDoc.createElement("Nested_Element")
        nodGrandChild.Text = Result(i)
        nodChild.appendChild nodGrandChild
    Next i
            
    Node.appendChild nodChild
This will create an XML with the added nested nodes having no indentation :
     <Block>
    <Element1>XXX</Element1>
    <Element2>YYY</Element2>
    <Element3>ZZZ</Element3>
    <Nested_Elements><Nested_Element>AAA</Nested_Element><Nested_Element>BBB</Nested_Element><Nested_Element>CCC</Nested_Element></Nested_Elements></Block>
To Fix the indentation add this piece of code (from stackoverflow https://stackoverflow.com/a/37634549/14360302)
Set xslDoc = New MSXML2.DOMDocument
xslDoc.LoadXML "<?xml version=" & Chr(34) & "1.0" & Chr(34) & "?>" _
        & "<xsl:stylesheet version=" & Chr(34) & "1.0" & Chr(34) _
        & "                xmlns:xsl=" & Chr(34) & "http://www.w3.org/1999/XSL/Transform" & Chr(34) & ">" _
        & "  <xsl:strip-space elements=" & Chr(34) & "*" & Chr(34) & " />" _
        & "  <xsl:output method=" & Chr(34) & "xml" & Chr(34) & " indent=" & Chr(34) & "yes" & Chr(34) & "" _
        & "            encoding=" & Chr(34) & "UTF-8" & Chr(34) & "/>" _
        & "  <xsl:template match=" & Chr(34) & "node() | @*" & Chr(34) & ">" _
        & "    <xsl:copy>" _
        & "       <xsl:apply-templates select=" & Chr(34) & "node() | @*" & Chr(34) & " />" _
        & "    </xsl:copy>" _
        & "  </xsl:template>" _
        & "</xsl:stylesheet>"
xslDoc.async = False
Set XmlNewDoc = New MSXML2.DOMDocument
XDoc.transformNodeToObject xslDoc, XmlNewDoc   'Line to fix indention
XmlNewDoc.Save XmlFile