<flsProSoc>
	<Accesso>
		<Identificativo>2008123456789000</Identificativo>
		<Erogatore>
			<CodiceIstituto>010ABAZZ</CodiceIstituto>
		</Erogatore>
		<Entrata>
			<Data>2008-03-18</Data>
			<Ora>09:30</Ora>
		</Entrata>
               <!-- 
               many other XML elements and subelements XML
               --> 
	</Accesso>
	.......................................
	.......................................
</flsProSoc>

DECLARE @max INT, @i INT, @xml XML
SELECT @xml=
          (SELECT * FROM OPENROWSET(BULK 
          'C:\MYXML.xml' 
          , SINGLE_BLOB) as x)
SELECT 
    @max = @xml.query('<e>
                        { count(/flsProSoc/Accesso) }
                      </e>').value('e[1]','int')            
SET @i = 1
WHILE @i <= @max BEGIN
	SET @xml.modify('
    insert element MyId {sql:variable("@i")}
    as first
    into (flsProSoc/Accesso[position()=sql:variable("@i")])[1]
    ')
    SET @i = @i + 1        
END