My sparse knowledge of the underlying O/S would lead me to believe that you cannot, due to the interdependence of other components.
On the other hand, considering that EMEM is (or appears to be) the last module of the boot sequence, it may well be possible (some esoteric command buried down in the vxworks shell).
When the 3300 first emerged our Mitel acct. exec. was quick to causion us that this 'in-the-skins' flavor of voicemail was going to be pretty basic.
And though improving, it's still limited in both features and capacity; this probably needs to be restated from time to time.
We may get there, but if you need the horsepower of something like NuPoint, Express Messenger isn't it.