I am writing my output to multiple files. However, I need to write my
output to existing files that have already been created. In Saxon I get
 an error:

XTDE1490: Cannot write more than one result document to the same URI, or
write to a URI

Simple question: How can you append to an existing file?

It's possible though that you might be able to restructure your
transform to write out the file you want to append to all in one go at
the end of the transform - it depends on what you are doing, but it
might mean holding your entire input XML in memory at once.

With Saxon you can use xsl:message with a certain incantation[1] to
achieve what you need (or from Java roll your own MessageListener to
do it)

--
http://andrewjwelch.com