I estimate that bunk beds take a little bit more than Y seats. Given a seat pitch of 32-34 inches, the space of two seats in a column is just not enough for the length of a bed. With two levels of beds we would need a little bit more total space.
My assumption was that there is some spare space in the cargo area. This might be wrong.
Anyway, I miss innovation in the area of economy class comfort, C and Y are moving apart. Certainly there is something possible. Passeners could be "stacked" horizontally with some density, utilizing the cabin height. There could be common seating areas for the meals, allocated on reserved time slots.
Airbus, hire me, I will draft something nicely for the A380!