that would actually be up to css.
lightboxes are just divs that are positioned absolutely (they move with the page) or fixed (they are positioned relative to the browser window.
Basic Lightbox HTML
<div class="lightbox_wrapper">
<div class="lightbox">
the lightbox content loaded by ajax
</div>
</div>
Basic CSS for a scrolling lightbox
div.lightbox{ height:250px; width:250px; margin:auto; position:relative; }
div.lightbox_wrapper{ height:250px; width:100%; top:200px; left:0 position:absolute; }
Basic CSS for a viewport fixed lightbox
div.lightbox{ height:250px; width:250px; margin:auto; position:relative; }
div.lightbox_wrapper{ height:250px; width:100%; top:200px; left:0 position:fixed; }
Now I believe that most of the common lightboxes make you include their css, or they add it to the DOM on load. If they as you to include a css file then just look for the class that declares the properties of a lightbox and change the position method. otherwise you'll have to add the values to your own css and declare them as important like this.
CSS property marked as important
div.lightbox_wrapper{ height:250px; width:100%; top:200px; left:0 position:fixed !important; }
as for another kind of lightbox, I haven't seen one so you'll have to explain more in a comment below...