Well there never a guarantee of miracles. But wheat and barley didn't start out as marginal soil crops.
And kangaroos are adapted to low calorie food and low water now. If you bulked them out to be 'better', that is bigger, fatter, more tender, they would need more calories for fat and more water to support more mass. So, no, there is no "all we need to do...".
But when your attitude is 'these brown native people would never figure this out' perspective, it's easy to think in terms of "all we need to do...".
Actually wheat and barley did start out as marginal so it crops, but this is irrelevant to modern agriculture. The reason we can feed the almost 8 billion people on the planet is that we have selected for strains that respond to fertiliser and agrochemicals.
The selective breeding for improvement of kangaroos is not a binary choice - you can improve for multiple traits and balance for multiple requirements.
Where in the hell have I even hinted at “brown native people would never figure this out”? This is frankly very offensive.
Good thing kangaroos are adapted to low calories food and low water. Now all we need to do is make them a better food source than they are right now.