Not hassle-free, but it is possible
a, Sign up for a US PSN account (search on Google for details)
b, Sign up for an account with ENTROPAY at
www.entropay.com
This is an online 'Virtual Visa Card'. You top it up with a Visa credit/debit card for a small fee (5%...about 50p for every £10). Go back to PSN and add a new credit/debit card...type in your Entropay account name, expiration date, card number etc and it's done! Now you just top up your Entropay account whenever you want to top up your PSN Wallet to buy stuff from PSN
I keep my Entropay money at a minimum. I just log in, add £10 or so onto Entropay and it's done immediately. Go onto PSN and select something to buy it. I add the funds to my PSN Wallet and it's taken out immediately. Works flawlessly for me and I've probably spent about £30 using this since I started