My understanding is that this is not possible using Structure tags. The navigation works off the hierarchy.
Best option is to use jQuery to manually insert the navigation item. Here's a rough how-to:
Set your {exp:structure:nav}
tag to use add_unique_ids="yes"
. Let's say this outputs:
<ul>
<li id="link1"></li>
<li id="link2"></li>
<li id="link3"></li>
</ul>
Then use jQuery to insert your new nav link where appropriate. Something like this:
$('#link1').after('<li id="inserted"></li>');
This will get you this:
<ul>
<li id="link1"></li>
<li id="inserted"></li>
<li id="link2"></li>
<li id="link3"></li>
</ul>
There may be a jump in the nav list as the item is inserted so I'd suggest hiding the UL with CSS and then showing it after the insert with JS... something like:
ul {
display: none;
}
$('ul').show();
You'll need to adjust the selectors to work with your code.