Yes, but you will not be able to dock them and use the remote of the unit. 2 possible options exist.
Option 1: If your MP3 player does not support DRM, you may select to connect it to the USB Port, press SOURCE once or more to select USB mode.
Option 2: Use the 3.5 mm cable supplied with the device and connect the headphone jack of your MP3 Player to the Aux input on top of the unit. Press SOURCE once or more to select AUXmode.