How can I use iPod stereo boombox with charger to regular MP3 player?

I want to charge cheap player while playing in docking boombox, which has only one iPod adapter. Where can I get adapter (USB port) from iPod's dock? I want to sell my iPod and will use old Creative mp3 player. BTW, my boombox is Panasonic SC-HC20 Compact Stereo System.