is this really possible?

Its not possible for someone to strongly affect their metabolism by thought. Metabolism is primarrly controlled by hormones (primarily thyroid hormone), and the only real way that your brain can regulate this is through the hypothalamus, which is not under conscious control. Unless there are journal articles demonstrating that the hypothalamus can be regulated by the conscious mind I dont think its possible.

Men have caused themselves to exhibit signs of pregnancy (as have women) simply by believing that they were pregnant. This requires pretty drastic changes in hormone levels, which the hypothalamus regulates. So it is possible. It's not a matter of conscious control, however, so much as subconscious. It only happened because they thoroughly believed it was happening, not because they pretended it was.