Surely, we must know how to do this. I mean, we know how grass-eating ruminants break down the cellulose to obtain energy with various enzymes and whatnot. We could probably invent some kind of exo-stomach to pre-digest grass into an edible state. :)
Starch and cellulose are both glucose polymers. The only difference is the way in which the glucose units are attached to one another. Existing enzymes can only cleave one or the other. Ruminants don't actually have the necessary enzyme, but instead rely on bacteria in their stomach to do this for them.
I guess if you really wanted to, you could take cellulase in the way that people can take lactase to mitigate the effects of lactose intolerance.
How you'd actually get your stomach to brew that up into anything useful in time is anyone's guess, and what it would do to the rest of you is an exercise for the keen experimenter.
We are basically too active and too large to eat grass, even if we had lactase.
