What you want to build is called a Call Routing Annoucement (or CRA)...
You would record the announcements in your Administrator Mailbox, the one that says "The hard disk is XX percent full" when you log into it, there is a hidden menu option 9 that takes you there, and you can record custom recordings, make sure you look in the voicemail first. Say 010 for day and 011 for night, but check the programming form first to see which ones are used.
Then you add a CRA in voicemail applications, open that and go to digit translations and edit the entries as needed, then change the greetings with the numbers of the greetings you recorded.
Then you can call the extension number that was created to test it, once you are satisfied with it, change the ring-in point for day (and optionally night) to that extension number for that trunk group or DID translation.
There are multiple tutorials online to do this, and I believe several youtube videos. Denwa on youtube has great Mitel 5000 (Mivoice Office 250) videos.