I guess the first step would be to set up a PAL account.
However, I don't think the PAL store has the option to use PSN cards as funding like the US store does, I haven't looked for some time but I know when I set up my US account I saw the option, but didn't when I set up my UK one, which was first... This was a while ago, though. And we get basically no communication from Sony like the Playstation Blog, that's US only, so even if they did decide to release PSN cards I doubt anyone would even notice. (By this paragraph I mean I doubt very much we'll ever get PSN cards... Forgot to actually make that point)
I believe Paypal does this thing that lets you pay for stuff using Mastercard (you set up a temporary one or something?) or whatever, not sure how it works but maybe you could use that to pay for stuff. I dunno.