To get? You probably can. But to set? You can't, JavasSript is used for client side scripting on the browser, and cannot access a Session object from a server. JavaScript runs in the clients browser, your Session object is exposed by the server. There's disconnected space between them.
However, you can connect JavaScript to a web service, and inside the web service you can return data from your source to the JavasSript method.
If you're lazy you could store the value in a HiddenField instead so you can easily access it in your JavaScript code. Or use the dirty way like this to get the value:
<script type="text/javascript">
var sessionValue = '<%= Session["Test"] %>';
alert(sessionValue);
</script>
The <%= variable %> is a databinding expression that binds server-side variable value to client. Note that that will only work if you are passing a value (get a value from server). It won't work for setting value from your JavaScript. That's why I still prefer using an AJAX call or a HiddenField.