The only way I know to do it, is to create a "Voicemail Transfer Box."
What I do is create a Call Director voicemail box, linked to a phantom hunt group that is call re-routed always to the voicemail pilot number, lets say 550.
I then setup the menu in the call director section. First, set the menu to accept the same number of digits as however long your extension numbers are. (If your extension numbers are 3 digits, this needs to be 3 digits.) I then tell the "multi-key" function of the menu to route to a voicemail box. Within this option, I tell it to route based on "gathered digits." You then need to record the call director greeting for this mailbox, something like "Please enter the extension number of the user whose voicemail box you wish to transfer the caller to."
Then when a caller wishes to go into the voicemail of extension 200, you conf/transfer to 550, enter extension 200, and hang-up immediately. The caller will hear 200's greeting and be able to leave a message.
If you have a User/Device license you can burn, I've been playing with setting this up as a phatom extension instead of a hunt group. That way it can be a Dss/Blf on phones, and cuts down on the number of steps...
Unless someone who has more experience with NuPoint has a better way....