Is it possible to do a PhD in Environmental Policy and Economics if I do not have an academic background in Economics?

Is it possible to do a PhD in Environmental Policy and Economics if I do not have an academic background in Economics (except for two papers in my graduate prog. one on Environmental economics and the other on Resource Economics which, I feel were very watered down)? I have an Graduate degree in Environmental Studies and undergraduate degrees in Biotechnology and Law. I am very interested in pursing a PhD involving Environmental Policy; and Economics I feel is an integral part of the discourse. What can I do to overcome my lack of a degree/ course on Economics in my academics career so far? Will writing papers that uses Economic theories and concepts help me in getting accepted for a PhD position? Will I have to pursue another graduate degree in Economics? I am from India, when education is very compartmentalized and not generally interdisciplinary or multidisciplinary (with few exceptions).