It's possible, but it's not worth it. Paypal checks billing address and makes a small temporary charge on your card when you first set it as a new method of payment. If your visa gift card didn't come registered to you or your address, you have to set that up with the gift card issuer before trying to add it to Paypal. If there's a set amount on the card, you will be out whatever small amount they test charge to your account. You may have to wait a few days, sometimes weeks for that pending charge to disappear before you regain the full balance of your gift card.
The easiest way to convert visa gift/rebate cards into money you can use right away is buying Amazon gift cards for yourself. Amazon won't check billing address, they'll just charge the card right away for the full balance you tell it to. Then you can add the Amazon gift card to your Amazon account and use it however you want on Amazon.
