Yes, when you purchase the Plasma Data Add-On (available in the Expand and Transform pricing plans).
Let's take the category "Food and Dining: Fast Food" as an example. MNcDonalds, Chipotle, and other fast food retailers will fall under this category. You may decide to change the name of this category to "Food and Dining: Fast Food Chains." You are able to change the merchant categories across all applicable Hydrogen products (i.e. Money and Cards).