XML : Append Serialized Object Into XML in C#

I have a XML structer like that

  <?xml version="1.0" encoding="utf-8"?>  <Product>    <ProductName>da</ProductName>    <PluginPath></PluginPath>    <Instances></Instances>  </Product>    

and i serialize my object to string.

  <?xml version="1.0"?>  <Instance xmlns:xsi="http://bla bla" xmlns:xsd="bla bla" UniqueId="d4820029b7d7">  <InstanceName>Instance MyTestPluginForm</InstanceName>  <Description>Test Plugin IW</Description>  <AddedDate>2016-10-19T11:05:10.7443404+02:00</AddedDate>  <LogSettings>  <LoggingLevel>None</LoggingLevel>  <LogFilePath /><MaximumSize>100</MaximumSize  <ClearAfterDays>7</ClearAfterDays>  <IsSaveActiviesToEventLog>false</IsSaveActiviesToEventLog>  </LogSettings>  <ProductSpecific/>  </Instance>    

So I want to append the second one in the Instances node in the first xml. But as you see both has xml definition on the top and after serializazion i got xmlns:xsi and xmlns:xsd attributes.

How to solve this problem?