This is one way of doing it.
Create the navigation entry with a blank parent navigation entry.
Edit the
forum_view
template and add this at the top:
HTML:
<xf:if is="$forum.node_id == 8">
<xf:page option="section">navigation_id</xf:page>
</xf:if>
Change the
8
to the forum node ID.
Change the
navigation_id
to the navigation entry ID.