This probably isn't possible. The engine of this game is WAY too old for this type of thing, plus you have to consider the game of Grifball and how it works in Halo 3.
1. A person picks up a bomb, this can be done easily. All you need to do is reskin an Oddball, and set it on the map.
2. Scoring and the bomb. The bomb itself would not be hard to recreate, but the explosion resulting from the area where you drop it off might be hard to recreate. Scoring is also a slight consideration. It probably is possible, just like a CTF game, to remake this part.
3. The Players. The players is probably the hardest thing to do, mainly because of the color of the person holding the Grifball. Considering Halo's OLD engine, it is probably not possible to change the color of a person.

All in all, Grifball can be recreated in a slightly different way, but it will never be as fun as the real game on Halo 3.