Haha, my bad... I'm not really that familiar with GRBL, and I'm not sure how simple of an answer I can give practically. It looks really cool and well written. But Looking at their source code on GitHub, it seems very unlikely that it will "just work" without some patching. Namely, it looks like the GRBL code expects to drive motor controls directly from microcontroller pins, while the Motor Shield abstracts these controls through a shift register.
On the bright side, I think all the patching would be more or less confined to stepper.c. But someone would certainly need to "know what their doing" to pull it off. It's very sophisticated code, and unfortunately, it seems to be optimized for speed rather than for portability.
Stuff like this would need to be translated into shift register manipulations. And this interrupt handler would have to be reworked accordingly. It's certainly not impossible, but it would take some work. It might be hard to meet the "must be less than 33.3usec" requirement of the ISR, but you only need to compute and shift out 12 bits of information... I'm really intrigued, but I don't have a 3-axis gantry setup to try it with, and I'm also swamped with other stuff.
The key information you need to be armed with is that this is how to load the Motor Shield shift register, and that the following picture is the bitmap of the two shift registers (from the User's Guide).
