One method would be to dial into the VM system, press *, enter an extension number, enter the PIN (you will have to know everyone's PIN), choose the menu, then the option to record the self ID message.
Another option would be to take a look at our 3CX Auto Self ID tool. It runs as a service and nightly checks for missing messages. It will use Amazon's text-to-speech engine to create the necessary audio files for you. Amazon must have a very large library of names because it does a surprisingly excellent job of pronouncing names. And because it runs as a service, you do not have to worry about keeping new extensions updated. It all happens automatically every night. That's particularly helpful in large or transitory workforces.