There was some dicussion on that here:
http://www.hiawatha-webserver.org/forum/topic/394
Here is a real-life example from my hiawatha configuration:
UrlToolkit {
        ToolkitID = addwww
        Match ^/(.*) Redirect http://www.daemonforums.org/$1
}
[...snip...]
VirtualHost {
        Hostname = daemonforums.org
        WebsiteRoot = /home/www/daemonforums.org
        UseToolkit = addwww
}
VirtualHost {
        Hostname = *.daemonforums.org, daemonforums.net, *.daemonforums.net, daemonforums.com, *.daemonforums.com
        WebsiteRoot = /home/www/daemonforums.org
        AccessLogfile = /var/log/hiawatha/daemonforums.org/access.log
        ErrorLogfile = /var/log/hiawatha/daemonforums.org/error.log
        StartFile = index.php
        ExecuteCGI = yes
        UseFastCGI = php
}