Can I use it just as a message center for callers to leave messages for my employees?

Yes, you do not need to connect phones to the extension ports in this case. Just have one phone on extension 10 so you can program it through that phone and record the greetings for the various mailboxes. Of course you can do that by calling in from the outside as well.

Program all extensions to forward their calls to voicemail so the unit doesn't waste time ringing a non-existent phone in case someone tries to dial one of the extensions.

You can forward all extensions to one common mailbox if you wish.