They should be fine on a healthy diet - the block plus a variety of healthy vegs (such as baby kale, broccoli, cooked sweet potato, organic carrot pieces, cooked squash, mixed greens, etc) and occasional healthy treats such as cooked whole grains, organic pumpkin seeds, berries, fruit, a few cooked chick peas, a few cooked beans, a piece of cucumber, etc.
Or if you are concerned, freeze the Oxbow adult blocks for later and feed Oxbow for young rats plus a variety of daily vegs and healthy treats Edit: I have never fed this but some people do
Some people feed their rats eggs ..... but eggs are very unhealthy.