[xsl] Appending to existing file

["J. Zhang" <j.zhang@xxxxxx>]

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?

Thanks,

jz