There's lots of handgrips that are designed to work with grip shifters. Enough so that I wouldn't mess with modifying something that wasn't Grip Shift specific. There's even an Ergon GR2-S-G that combines the Grip Shift style hand grip with a small bar end.
If it was my bike, I'd install some kind of bar end to give you another hand position. The ability to rotate your wrists 90 degrees will greatly improve your comfort on longer rides. Since you said you have riser rides, I'd recommend a Cane Creek or Singletrac Solutions bar end because they don't look so goofy angling out the sides.