As for SignOn.com, the mechanism is to 'add' a personal card to the existing password protected account. It's not easy to find the function, you have to navigate down through 'Account Settings -> Authentication Settings -> Add an Information Card'.
It's the logical place for the function (along with changing password or adding a cert), but it's not a page users will see often. That's probably appropriate as it won't be a common operation but maybe a link from the main sign-in page would be useful.
I tried adding the same card that I added to my SignOn.com yesterday, but was hit with the following
I saw the same error when I tried to log-in using the registered card (thinking that the addition might have been successful notwithstanding the error). No luck.
The issue is also pointed out in comments on the JanRain announcement page.