Not really, no. Designing headphones for quality audio is very difficult, involves a lot of physics, and requires a purpose-built shell. You can try to replace the speaker for a better one, but adding a second speaker will only create interference, unless the shape of the headphones were designed to accommodate multiple speakers. Buying quality extra speakers will probably cost more than buying a whole new pair of headphones, anyway.

If you want to get better audio out of your existing headphones, applying a custom EQ is probably your best bet. Compensate for the lacking frequencies by boosting them, dampen the ones that are too pronounced. You can’t use software to compensate for the lack of frequency range of your hardware, but you can make the problem less obvious.