I'll assume your query produces a single record with the XML data type, and as the UNION ALL expects data types to match, and that my idea assumed a different scenario, this won't quite work as written, however..

You could use your query as a subquery as follows:

SELECT '<?xml version="1.0" encoding="iso8859-1"?>' +

(

SELECT doc.col1, doc.col2, doc.col3

FROM TestTable AS doc

FOR XML PATH ('doc')

)

Let me know if that works for you...