require 'nokogiri'

doc = Nokogiri::XML(IO.read("D:/Games/Games/test.xml"))
selectedFields_node = Nokogiri::XML::Node.new("selectedFields",doc)
selectedFields_node.content = "test1"
logDefinition_node = Nokogiri::XML::Node.new("logDefinition",doc)
logDefinitions_node = Nokogiri::XML::Node.new("logDefinitions",doc)
server_node = Nokogiri::XML::Node.new("server",doc)
advancedLogging_node = Nokogiri::XML::Node.new("advancedLogging",doc)
systemwebServer_node = Nokogiri::XML::Node.new("system.webServer",doc)
location_node = Nokogiri::XML::Node.new("location",doc)
location_node.add_child(systemwebServer_node)
systemwebServer_node.add_child(advancedLogging_node)
advancedLogging_node.add_child(server_node)
server_node.add_child(logDefinitions_node)
logDefinitions_node.add_child(logDefinition_node)
logDefinition_node.add_child(selectedFields_node)
doc.root.add_child(location_node)

file = File.open("D:/Games/Games/test.xml",'w')
file.puts doc.to_xml
file.close