Having the calls from multiple extensions go into one voicemail, isn't a problem. Having users change which one to use, on the fly, is. The other issue might be the message alert.
Whatever VM box you use will be associated with an extension number, it could be a "Dummy" extension with no physical set, or one of the existing extensions. however where calls go, when unanswered or busy is pretty much determined (limited ?) by the forwarding rules in the extension settings, or in the Ring Group, if used.
Unless multiple emails are sent, one to each user, to alert of a new message, then a line key, for the extension used for VM, and a message waiting light, would have to appear on each users set.