First, you're going to have to define what poor sound means to you. Modding your closed back headphones to add more speakers isn't going to make them sound better, but probably worse.

The best way to get better sound is to replace your headphones with better headphones, or to use a better source when playing music. Even better headphones aren't going to improve the quality of music if you're playing lossy Mp3s.

Everything about a headphone design influences its sound, not just the driver. If your headphones are crappy, chances are that changing out the driver won't help much, even if that was feasible which it is likely not.

Don't listen to these chumps. More is always better with speakers. I wear my headphones over my ear buds and sync them up. It sounds amazing and everyone says I am the bomb.

Half the battle with headphones tends to be isolation. If you have decent sounding speakers but bad isolation you have mediocre at best headphones. Typically replacing the ear cups can be an option but adding speakers likely won't yield additional quality. Unfortunately if the speakers that already are in the headphones aren't up to snuff there won't be a whole lot you can do. It's important to make sure we're talking about the actual headphones though and not the audio amplifier or the source audio.

Short of just getting new cans because the speakers are just that bad, I think you'll be better off enhancing the sound source instead (EQ, DAC, etc.)

Why are you trying to get an answer via an internet forum instead of in the obvious way?