From management console, it does not allow you to import a private key. Sometimes if it is mandatory to pre-generate the key from another sources, you will not able to use the pre-generate key.
However, replacing the private key and certificate is not that difficult. They are located at
assuming the installation folder is /opt/arcsight/logger.
The private key file, must be not encrypted (-nodes), is called server.crt. The certificate file is called server.pem.
Replacing these two files with your own pre-generate private key and certificate, then restart the HP ArcSight Logger service. This makes the newly loaded private key and certificate effective.
This is not officially supported by HP, I think.