xf:insert-document is an extension used to insert an external document between two fo:page-sequence elements; it can be applied only to fo:root.
The following example inserts CoverPage.pdf at the beginning of the document:
<?xml version="1.0" encoding="utf-8" ?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"
xmlns:xf="http://www.ecrion.com/xf/1.0"> (1)
<fo:layout-master-set>
<fo:simple-page-master master-name="all-pages" page-width="8.5in" page-height="11in">
<fo:region-body region-name="xsl-region-body" column-gap="0.25in" padding="6pt" padding-bottom="6pt" margin="0.7in"/>
<fo:region-before region-name="xsl-region-before" display-align="after" extent="0.7in" padding="6pt"/>
<fo:region-after region-name="xsl-region-after" display-align="before" extent="0.7in" padding="6pt"/>
</fo:simple-page-master>
<fo:page-sequence-master master-name="default-sequence">
<fo:repeatable-page-master-reference master-reference="all-pages"/>
</fo:page-sequence-master>
</fo:layout-master-set>
<xf:insert-document src="url(CoverPage.pdf)" /> (2)
<fo:page-sequence master-reference="default-sequence">
...
<fo:flow flow-name="xsl-region-body" font-size="12pt" font-family="Times New Roman">
<fo:block>...</fo:block>
</fo:flow>
</fo:page-sequence>
</fo:root>
Key observations:
(1) In order to work with the extension, the corresponding xml namespace must be defined.
(2) The inserted document's source must be specified.