It can still be done no problem at all!
Firstly to create your templates start as a HTML file and keep it at your /root directory inside your HTML create a container with an ID/CLASS and dynamically load all the content already existing in your website using jquery .LOAD(). example:
NEW WORKING TEMPLATE:
<html>
<head>
//load here your new css and js libraries
<script type="text/javascript">
$(document).ready(function() {
$(".main-content-new").load("/my-current-home-page-or-any-other-page-link .main-content-old");
});
</script>
</head>
<body>
<header></header>
<div class="main-content-new">everything on the page specified to load will load here..</div>
</body>
</html>
EXISTING TEMPLATE AND SITE:
<html>
<head>
//load here css and js libraries as normal
</head>
<body>
<header></header>
<div class="main-content-old">everything inside here will load to the other page..</div>
</body>
</html>
REMEMBER YOUR NEW HTML HAS TO SEAT ON THE OLD SITE ONCE EVERYTHING IS IN PLACE YOU CAN START AND MIGRATE THE NEW TEMPLATE TO THE /Templates FOLDER! and USE {tag_pagecontent} Module to load the rest.
This works in the eCommerce way.