Not sure there is a procedure to do it for you but you can come up with one using tdom by following the example below:
set records {{{Ayn Rand} {Atlas Shrugged}} {{Leo Tolstoy} {War and Peace}}}
set doc [dom createDocument "library"]
set root [$doc documentElement]
foreach rec $records {
lassign $rec author title
set el [$doc createElement "book"]
set field1 [$doc createElement "author"]
set text1 [$doc createTextNode $author]
$field1 appendChild $text1
set field2 [$doc createElement "title"]
set text2 [$doc createTextNode $title]
$field2 appendChild $text2
$el appendChild $field1
$el appendChild $field2
$root appendChild $el
}
puts [$doc asXML]
Here's what the output looks like:
<library>
<book>
<author>Ayn Rand</author>
<title>Atlas Shrugged</title>
</book>
<book>
<author>Leo Tolstoy</author>
<title>War and Peace</title>
</book>
</library>
Of course, you can just append to string. You could also use appendFromScript (which I prefer) but it is somewhat more cumbersome to setup.
