Take a backup, neitherway i dont ubderstand the need of backing up the old outbound certificate.
There is no way to export a certificate's private key with checkpoint products.
Just issue a new ca certificate for the ssl inspection and spread it to the organisation.
If the customer already has some ca authority in use within his network you can issue a certificate from the "subordonate certificate authority" template it will be more best practice and you wont need to speard the certificate in orser to use it.