So unless you can alter those stored trusted certificates (which you can't) or get a certificate that appears to be Sony's and sign by Verisign (or whoever signed their SSL certificates) you're out of luck - i.e. PS3 won't accept the incoming stuff as genuine and you end up with exactly the same message you have already received... One more option is to brute-force the keys
Not to ruin your enthusiasm but this is hardly going to work. The PS3 most probably (as browsers and OSs do) stores hashes of the trusted certificate authorities (like VeriSign for example). The certificate of Sony servers are signed by these top-level authorities and PS3 can verify that what comes in is really a Sony certificate. It maybe even stores Sony's certificates directly.