Mod Portable DVD Player to Include AV-Inputs?

How difficult would it be to add a/v input into an existing device (such as a portable dvd player) with currently no a/v inputs, but only a/v outs?
My reasoning is that I received a hand-me-down portable player, but I was under the impression that it could handle A/V in by default; so I was about to plug in a Playstation (or other game console) into it. Understandably, I could sell the portable player and buy a portable screen with money to spare, however in my case, it's a matter of respect for the party that gave me the said device.
Any EE's around?