I know something about cryptography (I have already used it my job for some comm tools for my clients), and what you write seems strange to me. The addons seem not to be encrypted at all - they are signed. The difference here is for decryption you need a private key, but for signature authentication the public one is enough. You can reverse engineer the "signature" verification as much as you want, and you should find what algorithm is used (most likely some RSA or AES variant?), but unless BS implemented something very wrong (which is possible, jugding by how many bugs they have in their games), you will not get a private key, and you will therefore not get a way to re-sign the addon.
On the same principle all e-mail digital signatures are constructed, or web site authentication. If you are really able to re-sign the modified addon, you could proly apply for a Nobel prize or something
, because on the fact this is not possible a lot of real-world security is based on this, and nobody was able to beat it so far (which is why most attacks are based on exploiting SW bugs unrelated to the crypto algos, or by exploiting human factor, which is most often the weakest link).
Just my 2 cs.