In the DialogHandler.onSelectCertificate(CertificatesDialogParams params) method you can select not only SSL certificate from the given list of certificates, but also select SSL certificate instantiated from standard Java X509Certificate class. For example:

browser.setDialogHandler(new DefaultDialogHandler(view) {
    @Override
    public CloseStatus onSelectCertificate(CertificatesDialogParams params) {
        X509Certificate x509Certificate = loadCertificate();
        params.setSelectedCertificate(new Certificate(x509Certificate));
        return CloseStatus.OK;
    }
});