How to open existing XML file and apend elements into this

I have many page and extracted data in each page but I want to write all data into 1 XML file
http://...page=~#PAGE#~
PAGE: session variable.(PAGE=1--> 10)
Thanks.

I specify the same filename ?

I used same filename but Xml file was overwritten .
Example
I have written script .
XmlWriter xmlWriter = new XmlWriter("./simple.xml", "simple-root");