You might want to consult an exterminator. The roaches are probably not coming in the bagged potting mix, but frequenting your porch, and finding potted plants a convenient place to hide.

Certainly, not storing the bagged potting mix on the porch would help keep roaches from coming into the bags; if you have nowhere else to store the potting mix, you could get a large sealed container, plastic or preferably metal, and immediately put the soil into that when you get it home. However, if there are roaches traversing the porch they could still get into your potted plants.

I don't know what kind of roaches you have but in the Midwest we have something called a Wood Roach. Not a German cockroach, it's very thin and brown (not dark brown to black like the German). They don't live in the house or the sewer drains - they prefer wood piles, mulched areas, gardens. I see them in my compost. Rarely see them inside the house.

You might look at a multi-pronged attack. Roach motels on the porch, maybe some diatomaceous earth for them to crawl through, and whatever else you can come up with. I don't know if there are any scents they don't like (or hot pepper spray) but you could search around for that. And of course there are sprays.

For some reason my early post didn't go through.

In the southern part of the United States there is giant cockroach that is harmless. They have been know to make their homes in moist mulch piles. I would say identify the species before you start worrying.

Hot pepper would not do anything to deter insects, BTW. Only mammals have the capsaicin receptor. Other critters don't have it (technically birds do have it, but it doesn't function the way it does in mammals, which is why birds happily eat hot pepper.)