Web Service host is requiring both a DER X.509 digital certificate and a PKCS12 private key

To consume a Web Service, the host is requiring both a digital certificate and
private key issued by them to access the Web Services API.
The .cer I can attach to the soap wrapper programatically with: System.Security.Cryptography.X509Certificates.X509Certificate.CreateFromCertFile(strCertFileLocation)

The private keyfile, in the PKCS12 (private key) format (.P12 file) is the one I am not sure how to handle. Since the Crypotography class only handles DER X.509, I don't know of a way on the DASP server to get the private key in place.

How would a situation such as this be handled withina site on DASP?