openssl pkcs12 -in container.p12 -out key.pem -legacy -nocerts -nodes

openssl pkcs12 -in container.p12 -out certificate.pem -legacy -nokeys -nodes

openssl x509 -inform der -in certificate.cer -out certificate.pem

This requires a combination of some of the earlier demonstrated commands:

First, extract the key from the existing P12 container:

openssl pkcs12 -in container.p12 -out key.pem -legacy -nocerts -nodes

If you don't already have the certificate in a separate file, extract that from the P12 container too:

openssl pkcs12 -in container.p12 -out certificate.pem -legacy -nokeys -nodes

Then, concatenate the certificate and the intermediate certificates:

cat certificate.pem intermediate.pem > certs.pem

Finally, merge everything back in a new P12 container:

openssl pkcs12 -export -out all.p12 -inkey key.pem -in certs.pem