Hi, perhaps I can help you. I had the same problem. From 2 Files (crt & key) it is a easy way to pkcs12 Files. You can just use the command:
openssl pkcs12 -export -out intermediate/certs/%certname%.pfx -inkey intermediate/private/%certname%.key.pem -in intermediate/certs/%certname%.cert.pem -certfile intermediate/certs/intermediate.cert.pem
The Out-parameter is the pkcs12-File, inkey is the private key of the client, in is the client cert and certfile is the Intermediate CA.
But i had problems. My nodejs server didnot authorized this client certificate. I use the intermediate cert as CA cert in my nodejs server. Any solutions for this?