From their
website:
If you have a dog that is currently registered with another registry, you can register your dog using the R-2 form. This form can be printed out, from your computer, through our Website—in the Form Section. Attach a photocopy of your dog’s certificate to the R-2 form and fill out your personal information, such as name, address, and telephone number on the bottom section of the form. If there is a co-owner, fill in his or her name (there is no additional charge for a co-owner).
I can't find anything that specifically says what other registries they accept (maybe it's all of them!). I'd try contacting them... There's an email address and phone number on their site.