You could register them in APR-American Pitbull Registry
How Do I Register My Pitbull That Has No Papers Or Pedigree But It Is Full Blooded?
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.