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.
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...".
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.
And I fail to see how some hypothetical bulked up kangaroo farm is going to be any solution. 'Selective breeding' kangaroos can't change "there's not many calories out here, and not much water" into a nice juicy steak that western palates will make economical. See: Masai cattle breeds.