Would you be able to provide the public portion of one of the PKI certificates for me to test? I would be willing to devote some time to testing it for you. We can import other types of certificates either in Symantec Encryption Desktop or on the server, so I could probably run it against my lab fairly quickly and give you a rough idea of whether it works, and how to set it up. If you PM me, we can work out the details.
If you are wanting to import the certificate into the server for emailing to an external person, the model of USB should't really be a factor. The type of certificate will be the issue.