You would have to create a new contact, maybe call it "calling card" then assign a single key speed dial. When you enter the 800 number for the card, follow it with either a hard pause, a 3 second pause or a soft pause, then your # and pin.
Before using a BB, I had a SE phone that would allow you to enter the calling card number into the phone. When you dialed, an option was to use the calling card. It was very handy if you needed it.
You can go to handango.com and look for BerryDialer. I was using a calling card for a while as I was doing calls to Canada often and the problem with using the calling card as a speed dial, you can't dial from the phonebook.
I didn't use BerryDialer and it is $40 which is kind of steep so you may want to do a google search. I think RIM should have included something especially for those of us that need to make international calls.
Apologies for really late response. I tried BerryDialer, could not get it to work. When I look in the call log, the string of numbers is correct, the BB will dial 800#, but nothing afterward. I can't tell if it's a BES config issue or not. BerryDialer CS not sure either.