require 'rubygems'
require 'nokogiri'

html = Nokogiri::HTML(DATA)
html.xpath('//table').each do |htable|
  tbody = Nokogiri::XML::Node.new('tbody', html)
  htable.children.each do |child|
	child.parent = tbody
  end
  htable.add_child(tbody)
end

puts html.xpath('//table').to_s

__END__
<table border="0" cellspacing="5" cellpadding="5">
	<tr><td>Data</td></tr>
	<tr><td>Data2</td></tr>
	<tr><td>Data3</td></tr>
</table>