You could register them in APR-American Pitbull Registry
Find the proof,blood test. Otherwise not going to happen.
You can go to Universal kennel club and and get a Foundation Stock Application this will get you started.
Unless you have papers (AKC / CKC) you can't register. I have a full-blooded Mini- Schnauzer but no papers! You only need them if you are going to breed & sell.