Hi,
The goal of PayPal express checkout is to skip the checkout of the e-commerce solution. And thus skip the address form, the shipping methods selection, etc of HikaShop and go directly to PayPal.
Besides, UPS requires not only the post code, but the country and the city, so even if you were redirecting to the Express checkout only after entering these and then selecting the shipping method, you would almost have half of the user info already entered. So it gets a lot less interesting.
Now maybe PayPal has a way to have UPS fees in your orders on their express checkout (you need to see that with them), but otherwise, the address information would have to be entered on the HikaShop checkout so that HikaShop can get the UPS fees.