actually, it is pretty simple to add a new CC as a new payment method. Just add new fields like kktype to it, the new field entries will be blobbed to your database (serialized, using your SALT-hash?).
BUT: Be aware that you cannot simply collect CC data on your server. Concerning German law (and I think EU law as well, cannot imagine it is different over there in IRL) it is not permitted to save that data except you own a PCI cert.
No idea why you get that error message on the help page. Just informed Lars about it.
Thanks - actually found it after I posted the above. As for storing the cc details I actually will talk to some payment processors what the law/requirements are. I think it is not as stringed in the UK as I know a couple of site that actually store your card details for repeating business convinence.