I don't quite get the reasoning behind this question. Do you want to enable grasses to store more water? And then use this stored water in the plant tissue somehow? I think even at the best of times, there would only be a tiny bit of water that could be unlocked by harvesting all of this grass, and even if you could, then this water is likely filled with other compounds that make it unsuitable to use as a water-supply, in the same way as with xerophytes.