Sounds like a repeat of what I went through when I bought my first house. Here's what took me way too long to learn because I'm a dumbass:

1. Water comes BOTH down from above and up from below ground
2. From Above is surface water and is controlled via grading away from the house, catch basins, surface drains and downspout extensions.
3. From Below is groundwater and is controlled by french (trench) drains and sump pumps.
4. Do not try to mix the two (putting surface water into french drains or a sump is a no-no.)

OK, now that I have that off my chest:

Yes, sure, add as much soil as you can next to the house as long as you don't get too close to your bottom course of siding or stucco weep holes. You don't want to cause more rot or other problems. If your soil is relatively impermeable (see below) aand graded away from the house, you won't have any problems with *surface* water going into the basement, even if you have gaps (but no amount of grading will help with groundwater, hence the sump pump).

The gaps are mostly a concern for foundation stability or for rodent/pest intrusion. Your foundation may not care if there are gaps depending on the design. If either of these are a concern than you don't actually want topsoil, you want the base to be some kind of engineered road base or backfill mix that will compact and will be less likely to wash away, expand when wet, and is harder for rodents to dig through.

Then on top of that, top soil but "Top soil" is a loose term. If it contains a lot of organic matter and sand, then water will go right through it until it hits something like clay or silt and may drain right back under your foundation. You want plain old DIRT with a decent amount of clay in it so that when it gets wet it becomes impermeable and the water runs off of it. One alternative is heavy plastic sheeting held down by rock, bark, sand, any soil, or small children too stupid to come in from the rain.