How to create a person doc in the PNAB with a X509 certificate out of a .cer file?

Hello,I want to establish an s/Mime communication with a customer. I have asked the customer to send us a signed email, so that we can retrieve the public key. He has sent us a .cer file instead, which I understand contains the X509 certificate. I would like to be able to create a person document in the PNAB for this customer, incl. the certificate but I do not know how to do this (I only know the procedure if the other person sends a signed email).

Can anybody provide some help?

Thanks.