"One-way SSL" (TLS/SSL without client certificate authentication)
only means that the server does not validate the identity of the client. The rest of the process is the same - encryption and validation of transmitted data is still two way.
This said, what is actually the purpose of the new certificate in PFX format? Is this the client's certificate (which means you would still use two-way SSL), or is it something else - such as the server's certificate (which should be without a private key)?
Of course, if the PFX file does actually include the private key, the Certificate.LoadPfx (or CertificateChain.LoadPfx) method should be able to load it. Which version of Rebex FTP/SSL do you currently use? We have enhanced this method a bit recently when adding MS CNG support, so it might be useful to try the latest release of Rebex FTP/SSL. If it still doesn't work, please let us know.