I have often wondered if it would be feasible to using either a blank sheet design and building it in or using an existing design to take a passenger jet and build a “bomb truck” out of it.
It's possible, but you'd want to build in the capacity from the start if you could.
Reason: you want the payload to be around the center of gravity/center of lift, so you don't become mistrimmed (or uncontrollable) when you start dropping it. Most large passenger aircraft, however, have wings mounted at the bottom of the fuselage, and so the main wing spars tend to run through that exact spot. You can't just cut a hole in the spars to let ordnance through, so you have to get creative with how the ordnance leaves the aircraft. Take a look at the Boeing P-8 Poseidon, for example - you'll note that it carries its ordnance in panniers below and beside the fuselage proper. It can get away with this because as a maritime patrol aircraft it doesn't need to carry much, but the same trick wouldn't work for something intended to carry lots of bombs.
Ideally you'd want something with a high-mounted wing and nothing else important around the CoG, so you've got space for bomb bays etc. Then again there was also a proposal to turn 747s into cruise missile carriers by dropping them out a side-mounted door, so what would I know?