My employee uses p12 certifcates for OpenVPN. On Manjaro Linux, the Network Manager does not support these directly, so I had to extract the CA certificate, User certificate and private key to be able to configure the VPN.
$ openssl pkcs12 -in cert.p12 -out userca.pem -cacerts -nokeys $ openssl pkcs12 -in $pfx -out usercert.pem -clcerts -nokeys $ openssl pkcs12 -in $pfx -out userkey.pem -nocerts
For the userkey I also had to set a password and supply it to the "User key password" option in the Edit connection dialog.