Whenever you do breathing exercises, use caution not to overdo it and make yourself dizzy or hurt yourself in any way. Anything that helps you gain relaxation control over your breathing should be beneficial to your harp playing. Meditation in its various forms and guises tends to emphasize controlled, often slow and deep breathing, sometimes even with chanting. Breathing exercises associated with meditation practice can be applied to breathing for the harmonica. Running, swimming, and other aerobic exercise can help strengthen your lungs and possibly improve your lung capacity, or at least allow you to more effectively utilize the lung capacity you have.
Most people breathe from only the top third or so of their lungs.
Most of the capacity lower in the lungs is not used, not well developed, and not contributing much to the person's breathing. We need to develop the lower parts of the lungs , breathing fully and deeply. Breathe out slowly as far as you can. Completely empty your lungs. Squeeze out the last bits of air.