DND (Do not Disturb) actually is just that - Do not Disturb me. Call Forward All is actually what you want.
Press <feature> <your DND button> and see what it displays on your LCD, chances are it shows <feature><60> only. Strictly speaking, for your desired action (call forward all) it is the same key sequence as DND, but with the addition of your voice mail pilot number.
So try <feature><6><0><your VMS pilot station><feature> and then ring that extension. I bet it goes directly to Voice mail.
If the above works, then you would need to reprogram all of your DND buttons to change into call forward all <to voicemail>
Hope that helps.