Powershell XML add key value as element

You can't do top-level member-access enumeration with $sample.TestTop.ChildNodes.td because you want to interleave td rows with their corresponding line = ... rows, so you'll need to iterate over the TestElements nodes yourself...

[xml] $sample = @"
<TestTop>
  <TestElements>
    <td field="head.office">ABC</td>
    <td field="head.number">101</td>
    <td field="head.colour">Blue</td>
    <key>
      <office>ABC</office>
      <number>101</number>
      <line>1</line>
    </key>
  </TestElements>
  <TestElements>
    <td field="head.office">ABC</td>
    <td field="head.number">102</td>
    <td field="head.colour">Red</td>
    <key>
      <office>ABC</office>
      <number>102</number>
      <line>2</line>
    </key>
  </TestElements>
</TestTop>
"@

$data = $sample.TestTop.TestElements | foreach-object {
   $_.td | foreach-object { [pscustomobject] [ordered] @{ "field"=$_.field; "#text"=$_.InnerText } }
   [pscustomobject] [ordered] @{ "field"="line"; "#text"=$_.key.line }
}

$data
# field       #text
# -----       -----
# head.office ABC
# head.number 101
# head.colour Blue
# line        1
# head.office ABC
# head.number 102
# head.colour Red
# line        2

$data | export-csv "myfile.csv"