Yeah this is the same thing I normally do when a customer wants a general mailbox. Just pick an unused station port and use it's mailbox as general and assign a flex button to open it.

You can also create a virtual extension "VEX" for the mailbox under Device Port Num Change(101) and click Virtual Registration in the top right. Then select Device Type: station, Device ID: VEX, Virtual MAC: Yes and click register. It will create a virtual extension on the system that you can view under Device IP Plan(103). I think the only reason you may want to do it this way is if you don't have any physical station ports available.