using keypairs stored in a keytool database

Is there an easy (or possible) way to use a certificate/private key that is stored in a java keytool database to sign xpi files?
I already own three verisign certificates and I would like to use one of them to sign installs for firefox.
I guess I'm looking for some steps to extract a pkcs12 file from the certificate store and put them in the correct place for the nss signtool.
I also have a certificate stored in the netscape keystore, I could get the pkcs12 file out of there. Will a netscape certificate work for firefox?

Thanks,
Steve