I assume that you mean you'd like to do your post-OW certification with PADI. If this is true, there's nothing you need to do other than let your PADI instructor know that you want to take a class through them, and show your c-card if the instructor doesn't know you already. I got my SSI OW certification and my PADI AOW certification. PADI doesn't cross-check with other agencies that you are already certified; ensuring that you are already certified is the job of your instructor.
However, if you want to get your OW certification with PADI even though you already have your SSI certification, you will probably need to re-take the class. I'm not sure why one would want to do that, so I'm going to assume (unless told otherwise) that this is not what you are talking about.