convert self-powered speakers to passive
Most headphone amplifiers are designed to drive at least 8 Ohm load. The speakers can have different resistance, but 4 and 8 Ohm speakers are quite widespread, and 2, 16, 60 Ohm or even more are rare.

If your speakers' resistance is at least 8 Ohm, it's possible to use them with your MP3 player just like headphones. The use of this application is another question - the volume will be so low that you will need to hold the speakers close to your ears, just like headphones :)