Hi,
You can log-in a customer to paypal. They can use the "auto-login" of paypal but you can you, force a login.
Otherwise it means that you will store the paypal username and the paypal password of your customers...
There is nothing to do in HikaShop ; it's just not something possible with Paypal.
If you want to continue investigating, I will recommend you to contact paypal directly.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.