You can send an outside call directly to a mailbox with Feature 14 and the extension number (put Feature 14 on a button and label it "Voice Mail Transfer")
You can send a voice mail message to another mailbox by logging into your own mailbox and pressing 1 to send a message (if you have Partner Mail or Partner Messaging, NOT Partner Mail VS or Partner Voice Messaging PC Card)
If you have a message in your own mailbox you can forward it to another mailbox, by pressing 12 during or after playback (again, only Partner Mail or Partner Messaging)
The only thing you cannot do is pick up your handset, and send a call directly into a mailbox, unless the other extension is in Do Not Disturb