You can do that if you choose, but it is not as effective as drilling the slab inside the garage. This is because the garage is a separately poured slab, meaning the termites could come from deep underground and come up to the surface under the garage slab, and enter the wall where your door is going into the house untouched. You don't have to drill every wall in the garage, just the walls that are not exterior walls, where you have living space on the other side of the garage wall and apply Termidor. So it is a personal choice, but drilling the garage walls is the best way to protect all entry points.
To treat subterranean termites we always recommend to trench where you can and also treat the crawlspace. Termidor SC can be used in crawlspaces. There are complete instructions on the Termidor SC product label (page 5) on how to use the product in the crawlspace, so if a bit inadvertently goes into the crawlspace it should not pose a problem.