How to add an existing key to the certutil key database

No, certutil doesn't have an option to add private keys. You need to use pk12util for that.

If your private key is in PKCS12 format, you can add it to the key/cert database with

pk12util -i keyfile.key -d/path/to/database -W password

If it's in PEM format, you'll need to convert it to PKCS12 first by

openssl pkcs12 -export -out server.pfx -inkey server.key -in server.crt -certfile CAcert.crt

then import server.pfx with pk12util as above.

Unfortunately certutil and pk12util often don't come with man pages, but certutil -H and pk12util -H provide some help. There are also some online docs and man pages.