open checkout in new overlay/pop-up window

is it possible that after I added my products to the cart, the checkout page (with the login/registration forms etc...) opens in a new overlayer or popup? An overlayer like you have in the hikashop demo after adding a product to the cart

There is no option for that.
You will have to edit the file "notice" of the view "checkout" via the menu Display->Views in order to remove the code there and add instead:
<?php
$params = '';
$js = '';
echo hikashop_getLayout('checkout','step',$params,$js);


Also, you will have to add the line:
<input type="hidden" name="tmpl" value="component"/>

after the line:
<input type="hidden" name="ctrl" value="checkout"/>

in the file "step" of the view "checkout".