What happens to your mortgage if you sell your home for less than you owe?

We would like to move into a bigger home. We live in a small condo that we paid a lot for. We will never get as much for it as we owe for our loan, but we have out grown it. We are not in a foreclosure situation because we are making the payments on time without a problem. If we sell our home for less than it is worth, can we add the balance of our mortgage onto the mortgage of a new home, or will we owe the bank the balance right then and there?