You can use the Include component to include arbitrairy content from non-Wicket locations, such as static HTML headers, or even do page integration from external sites.
This component is not used for normal page composition tasks. You should usually get by using Panels, Borders and Markup inheritance.
<span wicket:id="include">
Inclusion contents to be inserted here.
</span>
public IncludePage()
{
add(new Include("include", "to_be_included.html"));
}