Hi,
I'm not sure solution you want to use, but if you use that plugin (
www.yireo.com/software/joomla-extensions/paypal-access
) which allows the users to login on your joomla website with their paypal account, the username/password won't be stored on the website. Instead the authentication of the user will be done by contacting PayPal servers directly during the login process.
The address could then potentially be filled in HikaShop but that won't be the case by default. It would require another plugin to get the data of the address which is stored in the user session by that first plugin and then store the address data in HikaShop.
Tht could potentially be done by a third party developer.
Now maybe I didn't understand what you're talking about and you're talking about somehting else ?