May one learn how to breathe from the diaphragm (Buddha Belly Breathing) holistically throughout all of their activities and if so how exactly?
Welcome to the community! Before answering your question, I believe some clarification would be helpful.
As the user Buddhism pointed out, specific breathing exercises are more related to other ascetic practices of ancient India. One important component of the Buddha’s discovery of the path to awakening, is the leaving aside of exercises such as these. What the Buddha taught instead was the mindful watching of the breath, as well as the development of the four establishings of mindfulness.
But leaving that aside, to answer: yes, it is possible to maintain a meditative state throughout the day, and it was something that was actually taught by the Buddha. Though I do not practice to that extent myself (I find that it requires a great deal of mindfulness, which I do not have yet), it was the way the Buddha encouraged his disciples to practice, and it is my understanding that modern monastics are encouraged to practice in this same way as well.
I hope this is helpful, and Metta.