gistfile1.rb

require 'nokogiri'

src = <<-XML 
<office:text 
  xmlns:office="urn:oasis:names:tc:opendocument:xmlns:office:1.0" 
  xmlns:text="urn:oasis:names:tc:opendocument:xmlns:text:1.0">
	<text:p>one <text:span>span</text:span></text:p>
	<text:p>two</text:p>
</office:text>
XML

dest = <<-XML
<office:document-content 
  xmlns:office="urn:oasis:names:tc:opendocument:xmlns:office:1.0"
  xmlns:text="urn:oasis:names:tc:opendocument:xmlns:text:1.0">
    <office:body>
        <office:text></office:text>
    </office:body>
</office:document-content>
XML

text_p_node = Nokogiri::XML(src).at_xpath('//text:p')
target_node = Nokogiri::XML(dest).at_xpath('//office:text')

target_node.add_child(text_p_node)

puts target_node.to_xml