I could be wrong, but i believe the items in the spark kiosk menu are hardcoded as HTML, check out:
resources/views/vendor/spark/kiosk.blade.php
And there's not really any way of hooking in to add a new item unfortunately - though i agree that would be a great feature for extensibility.
I'm not sure if it's totally possible (or a good idea), but you could potentially do some clever HTML and file manipulations when the package installs - but i think it's not that big a deal asking people to insert the menu item themselves when installing the package.
You could even add your item as a view that they can pull in using @include:
<!-- Last item in the spark kiosk menu... -->
<!-- Users Link -->
<li role="presentation">
<a href="#users" aria-controls="users" role="tab" data-toggle="tab">
<i class="fa fa-fw fa-btn fa-user"></i>
</a>
</li>
@include('yourpackage::nav_item)
Hope that gives you some ideas.