You can but I wouldn't necessarily recommend it.
The first problem is that guitar amp speakers are designed for a certain limited frequency. CD/MP3 players play a full 20hz to 20mhz and could potentially damage the speakers if you put too much bass through them.
You probably won't have any problems though as long as you keep the volume down.
Also, be sure to use a line level output from the CD/MP3 player and not the headphone output. The distortion from a headphone output can blow a speaker in no time, even at low volumes.