You cannot sell your home without discharging the mortgage. You cannot discharge a mortgage unless you either pay 100% of it off, complete an approved short sale or take a personal loan to pay off the difference.
Wrapping the remainder of one loan into another mortgage is almost impossible. First, the closings would have to be back to back in order to discharge the first mortgage. Second, banks are only supposed to lend money based on either the purchase price or the appraised value – which ever is less. That leaves no room to wrap in a past loan amount.