Currently wrappers aren't handled, so you'd need to manually insert it. So, the default code for external islands looks something like:
<?php
if (!defined('UBB_MAIN_PROGRAM')) define('UBB_MAIN_PROGRAM',1);
$style_side="";$tbopen="";$tbclose="";
echo "<table width=\"100%\">";
include("/path/public_html/forums/languages/english/portal_islands.php");
include("/path/public_html/forums/languages/english/generic.php");
include("/path/ubbcentral.com/public_html/forums/cache/search.php");
echo "</table>";
?>
For example, here I'd have to do something like (basically copying the opening closing table wrappers):
<?php
if (!defined('UBB_MAIN_PROGRAM')) define('UBB_MAIN_PROGRAM',1);
$style_side="";$tbopen="";$tbclose="";
echo <<<EOF
<table style="margin-bottom: 5px;" border="0" width="100%" cellpadding="0" cellspacing="0">
<tr>
<td width="3"><img src="http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/top_left.gif" width="3" height="2" /></td>
<td width="100%" style="background-image: url(http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/top_middle.gif); background-repeat: repeat-x;"><img src="http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/top_middle.gif" width="3" height="2" /></td>
<td width="3"><img src="http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/top_right.gif" width="3" height="2" /></td>
</tr>
<tr>
<td valign="top" style="background-image: url(http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/left_side.gif); background-repeat: repeat-y;"><img src="http://www.ubbcentral.com/forums/styles/images/drop_shadow_wrapper/left_side.gif" width="3" height="10" /></td>
<td><table width="100%" cellpadding="0" cellspacing="0" class="t_outer">
<tr>
<td><table width="100%" cellpadding="0" cellspacing="1" class="t_inner">
EOF;
include("/path/public_html/forums/languages/english/portal_islands.php");
include("/path/public_html/forums/languages/english/generic.php");
include("/path/ubbcentral.com/public_html/forums/cache/search.php");
echo <<<EOF
</table>
</td>
</tr>
</table>
?>
So as you can see, quite a bit of manual work is needed to duplicate the exact forum style. In version 8, hopefully we'll clean this up and make it easier to duplicate.