convert headphone speakers into loudspeaker?

You can, for a few seconds by taking the wires and forcing too much power into them, but then you'll cook the speakers.

The problem is the headphone speakers aren't designed for louder volume. So while you could wire a bunch of headphone speakers together, none of them would be capable of making much noise.

It might be possible to take the headphones apart and extract each driver and then design a wall of drivers then you would need to design a customized cross-over system that drives each driver to the proper level and output and integrates it with all the other drivers...a process that becomes more and more complicated depending on the amount of drivers involved and how different they are from one another...To do it right would take a considerable amount of talent and experience and then even at that you could expect the results to be mediocre at best,,,,

It's possible, but neither worthwhile nor a good idea.

One problem with this concept is that

you wouldn't get as much volume as with a regular speaker

no matter what you did to try to increase it.

You may as well try to build an automobile from several bicycles. You can carry more passengers, but you won't go any faster.

WASTE OF TIME EFFORT, HEADPHONE PRODUCES VERY LITTLE SOUND WAVE AND IT DOES NOT CONTAIN A CONE LIKE A SPEAKER.

not possible