I don't know much of anything about natal rats, but just a quick perusal through google, netted me this for people keeping their natal as a pet. This site looks like it's the one that knows what it's talking about over the others I found:
Your Natal rats’ diet is one of the utmost important aspects in their care. They are still considered wild animals, as they are not domesticated, and thus must be fed with great care. Seed mixes, and other commercial rodent diets (even lab blocks,) are not necessarily recommended. While a formulated lab block for rodents (such as Harlan Teklad,) may cover the majority of a Natal’s vitamins a minerals, they are mostly comprised of soy and corn, which are fillers. Natal rats prefer higher protein content than domestic rats (14% to 18%) or mice (10% to 14%) Natals generally like a 22% protein up to a 33% in their food. I use and recommend a high quality, grain free, dog kibble as a staple (such as Taste of the Wild, Before Grain, or Holistic Select,) supplemented with parakeet seeds (without sunflower seeds,) and millet spray. As with most rodents, fresh foods in moderation can be fun, healthy treats. A Natal rat’s favorite treats, however, are mealworms, wax worms, and millet spray. Refrain from feeding your Natals high fat, high sugar foods. Avoid anything that is caffeinated, carbonated, and alcoholic. Never give your pets candy (including chocolate,) avocados, or fruit peels!
For people keeping them for their herps, the answer was slightly different. I can only assume that it's a cheaper alternative, and just adequate enough to keep them decently healthy until needed:
"ASF need for approximately a third of their diet to consist of fresh fruits and vegetables. Offer a variety, (they like everything you might like and perhaps more.) The rest of their diet should consist of a commercial rodent chow such as "Mazuri Rodent Block" or similar. They also relish millet and other seeds such as sunflower seeds, pumpkin seeds etc. Glass water bottles designed for use with rodents are the only way to go. They will quickly chew holes through any plastic bottle."