Is it possible to have an inflated stomach in other to fake pregnancy, and if yes what are the side effects?

What? Is this a joke? Why would anyone want to do this?

I am considering adopting a baby in some few months time and wondering if there are drugs that I can be no to make my stomach big so that it will appear as if the baby is my biological child

Not that I'm aware of... why lie? It will catch up with you one day.