What Domino's does is not that unusual. Papa John's uses a blend of semolina, wheat flour and soy oil. It's apparently granular in nature and is called Dustinator. Randy does something similar with his American Pie dough, except that he uses semolina, cornmeal and flour. It gets worked into the dough to produce a marbling effect. I like it.
Adding semolina flour to all-purpose flour or bread flour should make the crust chewier. However, you shouldn't go overboard with it because too much will make the crust too chewy and it can lose a lot of its crispiness. The most common percentage I have seen is about 25-30% semolina. I had a New York style pizza that, according to the pizza maker, used 40-50% semolina. He used volumes (scoops) so I don't know what it was on a weight basis (although I don't think there would be a big difference). The pizza tasted perfectly fine and, had the pizza maker not told me, I wouldn't have been able to tell that there was semolina in the crust. The basic flour he used was a high-gluten flour. If you have a choice, you may want to get the fine semolina rather than a larger grind.