I think the radio is made by Clarion and looking at the rear side of a unit there is a square connector on the lower right side that seems to for a changer unit. I don't see a way to control it from the front panel but there may be some way to do it. After looking at some things on Ebay there is a product called Yatour Digital CD changer available. Do a search on Ebay for that and see if that would be off interest to you. It gives you several ways to hook up music sources.
Also do a Ebay search for 'Car digital music changer Bluetooth iPOD USB'.