If you don't have access to the server at Company A, then someone over there should be able to export it for you to a .pfx format. That is as long as when it was installed it was marked exportable. I would just get in contact with someone at Company A and ask them to export it for you and send it to you. Make sure you ask them what the password is they used when they export it. If they are unable to export it, then you will need to contact your SSL provider and get the cert re-issued. To do this you would need to give the certificate provider a new CSR (Certificate Signing Request.)
Here is some info on how to export and then import a cert in IIS, if you or the hosting company is unfamiliar with the process:
http://www.isaserver.org/articles/exportsslcert.html