declare @x xml

select @x = 
'<params>
  <param name="Calc">1</param>
  <param name="Month">6</param>
  <param name="Year">2009</param>
</params>'

select @x as x

declare @paramDef nvarchar( 200 )
set @paramDef = N'@x xml'

declare @xQuery nvarchar( 500 );
select @xQuery = 
N'<params>
	{/params/param[@name="Calc"]}
	<param name="beginDate"> {/params/param[@name="Year"]/text()}-{/params/param[@name="Month"]/text()}-1</param>
	<param name="endDate"> {/params/param[@name="Year"]/text()}-{/params/param[@name="Month"]/text()}-30</param>
</params>'

declare @sql nvarchar( 500 );
select @sql = N'
select @x.query(
''' + @xQuery + ''') as y'

exec sp_executesql @sql, @paramDef, @x = @x